I am sending this article along, from Monday's boston Globe, because it describes the use of encryption by the hijackers. According to the FBI and other government sources, the hijackers did not use any encryption technology, such as PGP, to communicate with each other by e-mail. Other articles suggest that the hijackers used regular accounts on AOL and Earthlink, just like everybody else.

One by one, they obtained photo identification. Two by two, they rented mailboxes and bought airplane tickets. Three by three, they took new apartments and visited bars. Five by five, they killed innocents by the thousands.

In the weeks before the World Trade Center was flattened, 10 men, later accused in the suicide attacks that originated in Boston, went into a whirl of synchronized activity designed to shield them from detection, and to prepare them for an evil mission.

Seen in isolation, the activities have the mundane feel of daily American life: getting a driver's license, buying airline tickets, checking into a hotel, renting a car. But when the activities are viewed in the context of Sept. 11, patterns emerge of men living quietly, separately and anonymously, then suddenly acting in concert, collectively pursuing a horrible goal on a tight timetable.

For the most part, the 10 Boston hijackers cloaked their moves - living and meeting in small groups, communicating by clandestine means. Some may have hidden behind stolen identities. But at other times - getting in loud arguments and skipping a court date for a traffic stop - their masks slipped and they engaged in careless or arrogant behavior that could have been their undoing.

Despite the occasional missteps, terrorism authorities say, the Boston group fit the profile of a ''sleeper cell.'' Members of such cells insinuate themselves into a society, living for months or years among their eventual targets, while preparing themselves for a vague mission whose details are revealed to them only at the last practical moment.

Whether the final orders came from Osama bin Laden, his underlings, or another source altogether, a trail of credit card receipts, airline, hotel, and rental car records, and of interviews and witness statements in the United States and Europe reveals a surge of activity by the 10 Boston hijackers that climaxed on Sept. 11. The FBI and other law enforcement authorities are trying to retrace the hijackers' movements, not only to connect them to the crimes, but also to hunt down coconspirators, sponsors, and commanders.

Obtaining papers for daily lives

Much of the known activity leading to the attacks involves Mohamed Atta, 33, who carried Saudi and United Arab Emirates passports and who, authorities believe, commanded the five-man takeover of American Airlines Flight 11. There also are indications that he oversaw the five hijackers aboard United Air Lines Flight 175, one of whom Atta mentored, and whom he called his cousin.

Atta came to the United States in May 2000, after spending more than eight years in Germany, studying urban planning, and forming an Islamic student group that investigators say doubled as a recruiting station for would-be terrorists. He spent the first few months in the United States undergoing pilot training at Huffman Aviation International in Venice, Fla., leaving little impression beyond that of a dour man with a thick billfold and a short temper.

When the investigation is complete, May 2 of this year may turn out to be an arbitrary starting point, but it seems notable nonetheless: That was the day Atta obtained a Florida driver's license using his address at the Tara Gardens condominium complex in Coral Springs, Fla. The license was long overdue; he had owned a car at least since July 2000, a Pontiac Grand Am he had registered using the address of a Huffman flight school employee where he and his protege and ''cousin,'' Marwan al-Shehhi, 23, briefly rented a room until they were evicted for rudeness. The owner of the flight school said he also had threatened Atta with expulsion if his attitude did not improve.

One week before Atta obtained the license, he had been pulled over by a deputy sheriff in Broward County, Fla. The reason for the stop has not been released, but Atta was ordered to appear in court on May 28. He did not show up, and a judge issued a bench warrant for Atta's arrest; however, authorities say no effort was made to find him. It was his only known brush with the law in the United States before Sept. 11.

Atta had an Egyptian driver's license, although it is not known whether it was valid. That license was not what he needed to carry out his plan, and the traffic stop may have prompted him to correct that problem. Photo identifications are required for check-in at US airports, and authorities say a foreign license or other identification might raise suspicions, particularly if it were issued by an Arab country.

Atta's move to get a US driver's license wasn't an isolated act. Five of the other Boston hijackers also moved, in late spring and early summer, to get legitimate, up-to-date, and commonplace photo identification.

Al-Shehhi, the man Atta called his cousin while they lived together in Germany and Florida, received his Florida license on April 12. Al-Shehhi was aboard United Flight 175. A second American Flight 11 hijacker, Waleed M. al-Shehri, 25, renewed his Florida license on May 4. Hamza Alghamdi, 20, who was aboard Flight 175, received a Florida photo identification card on June 26 and a driver's license a day later. Satam al-Suqami, 25, and Wail Alshehri, 28, both of whom were on Flight 11, obtained their Florida identification cards on July 3.

Lodgings with some anonymity

They needed those documents for the next phase of their preparations. As summer approached, most of the hijackers moved from the homes and apartments where they had been living for anywhere from a few weeks to as long as a year and into cheap motels, efficiency apartments, and short-term condominium rentals, places where the comings and goings of strangers was the normal way of things. As they moved to eliminate their fixed addresses, at least eight of them rented post boxes in four separate Mail Boxes Etc. outlets, increasing their anonymity.

Law enforcement officials and authorities on terrorism say South Florida's large immigrant population and transient neighborhoods make it a natural backdrop for drug runners, terrorists and anyone else who wants desperately to avoid notice.

''It's not a place where unusual newcomers seem very unusual,'' said Mike Ackerman, a former CIA operations chief now president of the Ackerman Group, a corporate security firm with offices in Latin America, Asia and Europe. ''Just about anyone can find a way to blend in if they want to.''

At least nine, and possibly all 10, of the Boston hijackers spent most of the summer at Florida addresses spread over a 40-mile area from Hollywood, just outside of Miami, north to Boynton Beach. For the most part, they lived in pairs or threes, some changing addresses every few weeks, as they blended into the landscape.

Meeting periodically in small groups in motel rooms and all-night restaurants, they also are believed to have kept in touch using computers with Internet access at local libraries. A librarian at the Delray Beach Public Library told police that at least one of the hijackers was a regular visitor.

The FBI has retrieved e-mail messages from the hijackers that date back as far as 30 to 45 days, according to an FBI official. The official said the e-mails were in English and Arabic, that there were hundreds of communications, sent to e-mail addresses in the United States and abroad. The hijackers did not use encryption techniques, the official said. The content of the e-mails has not been disclosed.

On June 21, Waleed M. Alshehri checked into Room B-308 of the low-cost Homing Inn in Boynton Beach, Fla., paying $286 a week over a month's stay. He had two companions, and authorities believe they were Satam al-Suqami and Wail Alshehri. Both obtained their Florida identification cards using the Homing Inn as their address.

Residents of the inn, where rooms and apartments can be rented by the day, week or month, said many of the guests are seasonal workers or visitors from other parts of the country looking to gain a foothold in Florida. Planted on US Highway 1 among a string of similar hotels, the low-rise and low-rent Homing Inn is thoroughly nondescript.

The inn's owner, who asked that his name not be used, said Waleed M. Alshehri was clean, well-mannered, unstintingly polite - and never once used the telephone in his room. Veronica Nacole, a 16-year-old Homing Inn resident, said she often saw him hanging around the inn's pay phone. ''That was the only weird thing about him,'' she said.

A waitress at a nearby Denny's restaurant said she had occasionally served late-night gatherings of five Middle Eastern men - two of whom she later identfied to FBI agents as Atta and al-Shehhi.

Meanwhile, several of the others had taken up residence in nearby Delray Beach. Records show that Hamza Alghamdi lived for a short time in Unit 1504 in the Delray Racquet Club condominium complex at 755 Dotterel Road, with one or two other men.

In interviews with the Globe, residents of the complex said they frequently encountered two tenants of Alghamdi's unit carrying gym bags and tennis rackets. They would have fit in well at the complex, which is built around the Rod Laver Tennis Academy, and where numerous foreign students take short-term rentals.

Maria Siscar-Simpson, who lives directly below the unit AlGhamdi was in, said she had a heated dispute with two of her upstairs neighbors three weeks ago. One banged on her door and demanded to enter her condo, apparently to retrieve a towel and some clothing that had blown off his balcony onto a roof outside her apartment.

Siscar-Simpson said the man spoke angrily in a heavy accent and tried to push his way into her apartment. She slammed the door shut, locked it, and activated her burglar alarm. The man left without incident, although Siscar-Simpson was shaken. ''Who goes ballistic over a towel?'' she asked. Nevertheless, Siscar-Simpson said the men were polite the next few times she ran into them, usually in the parking lot. She said she last saw them on Sept. 9.

It was one of several events in the weeks before the attacks that might have drawn authorities' attention to the men, but that ultimately did not.

From Aug. 26 to Sept. 9, two days before the attacks, Atta and al-Shehhi stayed at the Panther Inn in Deerfield Beach, where the owner said they were frequently visited by a third man who might have stayed with them. Before that, al-Shehhi had a two-month lease at an upscale condo complex called The Hamlet Country Club in Delray Beach.

The Panther Inn owners, Richard and Diane Surma, said they identified photographs of Atta and al-Shehhi for the FBI. They also provided to the Globe a copy of Atta's and al-Shehhi's registration, which al-Shehhi filled out using his name. He listed one of the Mail Boxes Etc. offices as his home address, and indicated he was driving a ''blue Chevy.'' He left blank the box on the registration form seeking a license plate number.

''They were neatly dressed. They made no noise. That's the kind we like,'' Richard Surma said. ''It's not like Hollywood, where you can easily spot the bad guy. They blended in pretty well.''

Al-Shehhi settled the account at about 9 a.m. on Sept. 9, and they left behind belongings whose significance would become chillingly clear after the attacks. Surma said the items included flight manuals, an eight-inch stack of high-quality aeronautical maps covering every state on the East Coast, a three-ring binder with notes and a protractor, three martial arts manuals, and a box cutter.

The Surmas discarded the flight manuals, maps and notes, but held on to the martial arts manuals and other items. Richard Surma had wanted to save everything, but his wife wanted to throw it all away; in the end, he kept a few items as a compromise. The Surmas gave the material to the FBI after watching television reports of the attacks and then approaching an officer with the Broward County Sheriff's Department who was parked near the inn.

''When I saw it on TV it blew my mind,'' Surma said. ''I thought the stuff would help, and it apparently did.''

Though it is not known when the hijackers received their orders, investigators are focusing on a July 7 flight that Atta took from Miami to Spain, where he also had spent time in January. A Spanish newspaper, La Vanguardia, quoted investigators as saying he had met with three or four unidentified Islamic extremists at a hotel in Salou, a beach resort near Barcelona. Atta returned to the United States on July 19, arriving in Atlanta.

At least one other hijacker also spent time abroad in recent months. Al-Shehhi arrived at the Miami Airport on May 2 on a Northwest Airlines flight from Amsterdam; it is not known where else al-Shehhi went or how long he was outside the United States.

The hijackers' activities after Atta returned from Spain are sketchy, but on Aug. 6, Atta rented a car from Warrick's Rent-A-Car in Pompano Beach, Fla. It isn't clear when he returned it, but he rented a second car, a 1995 Ford Escort, from Warrick's from Aug. 15 to Sept. 5. Atta extended the rental until Sept. 9, when al-Shehhi returned it with more than 1,000 miles added to the odometer.

As Sept. 11 approached, Atta also was honing his flying skills. Between Aug. 16 and 19, he rented a Piper Archer plane three times at Palm Beach County Park Airport.

One way, all cash, no questions

Then, on Aug. 25, Atta filled out an Internet form on the American Airlines Web site to open a frequent-flier account, a requirement for making reservations over the Internet at the airline's Web site.

The next day marked the start of a four-day period in which the 10 Boston hijackers bought tickets on the target flights - all in first or business class. They apparently never raised suspicions at either United or American Airlines, even though several of them paid cash for one-way flights. Cash purchases and one-way flights are both supposed to be red flags for airlines trying to stop terrorists. An FBI document detailing the flight purchases uses exclamation points to mark each of the one-way tickets the men bought on Flight 175.

Another link among the men that went unnoticed was the fact that Atta, three others on Flight 11, and two suspects on Flight 175 gave the airlines the same telephone number. The other three suspects on Flight 175 shared a second phone number. The 10th suspect, aboard Flight 11, listed no telephone number.

Their similar addresses also did not set off alarms. Four of the men on Flight 175 told United Air Lines that their addresses were postal boxes in two different Mail Boxes Etc. outlets in Delray Beach, Fla. And four of the men on Flight 11 told American Airlines that their addresses were two Mail Boxes Etc. outlets in Hollywood, Fla. The other two gave no addresses.

The cluster of ticket purchases began Aug. 26, when Waleed M. Alshehri and Wail Alshehri purchased tickets on Flight 11 with different Visa cards and the same Mail Boxes Etc. address in Hollywood, Fla. The next day, Mohald Alshehri and Fayez Ahmed bought one-way business class tickets on Flight 175, paying about $4,500 each and using a Mail Boxes Etc. in Delray Beach, Fla., as their addresses. The store owner said the men had rented the box in July.

One day later, Aug. 28, Atta typed in his new frequent-flier account number, 6H26L04, and used a Visa card in his name to buy tickets on Flight 11 for himself and Abdulaziz Alomari, using a second Mailboxes Etc. office in Hollywood, Fla, as their address. The same day, al-Suqami paid cash for his ticket on Flight 11, and al-Shehhi paid about $1,600 for his ticket on Flight 175. Neither gave an address.

Finally, on Aug. 29, Hamza Alghamdi and Ahmed Alghamdi paid $1,760 each for one-way Business Class tickets on Flight 175, using as their addresses the same Mail Boxes Etc. office in Delray Beach, Fla. They used the same telephone number as Mohald Alshehri.

The five men on Flight 11 clustered toward the front of the plane, with seats in Rows 2, 8 and 10 - two in pairs and one alone. The seating pattern on Flight 175 was not known.

The trek north to Boston

Their reservations made, the time came to begin moving from Florida to New England. It remains unclear when each of the 10 moved north, and at least some were still in Florida the weekend before Sept. 11.

Atta, al-Shehhi and a third man got involved in an altercation at the Shuckums Bar in Hollywood, where they drank heavily on the night of Sept. 8 and Atta loudly claimed he was a pilot for American Airlines.

The incident apparently was the result of a simple misunderstanding. A waitress, Patricia Idrissi, was ending her shift and asked Atta to settle the $48 tab for his five drinks of vodka and orange juice, and Al-Shehhi's five rum and Cokes. Atta took it as an insult and questioned whether she doubted he could pay. ''He was bothered that I was asking him to pay his bill,'' Idrissi said, explaining why she called over a manager.

Had the incident resulted in Atta's arrest, that would have triggered the bench warrant for his missed court date, and it could have led to questions about his activities. And if that had led to a search of his room at the Panther Inn, authorities would have turned up the aeronautical maps, flight manuals, martial arts books and the other materials that were found only after the hijackings.

At least some of the hijackers were in Boston by Sept. 6. At 2:15 p.m. that day, a white Mitsubishi sedan with one or more of the hijackers was driven into a garage at Logan International Airport, then left at 4 p.m., according to law enforcement officials and copies of garage surveillance records reviewed by the Globe.

The car had been rented at an Alamo office near the airport, said Cheryl Budd, senior vice president for corporate communications for ANC Rental Corp., Alamo's parent company. She declined to say when, or by whom, it had been rented.

The white Mitsubishi made two more visits to Logan on Sept. 9: arriving at 8:27 a.m. and leaving at 9:13 a.m; returning at 4:15 p.m. and leaving again at 5:39 p.m. The next day, Sept. 10, the car was back at Logan for a brief visit, from 4:25 p.m. to 5:05 p.m. The white Mitsubishi made its last entrance to Logan the morning of the hijackings.

By Sept. 10, all 10 hijackers were in place, though they remained split into small units that would attract minimal attention or suspicion.

That day, Atta and Al-Omari drove a rented Nissan Altima to Portland, Maine, and spent the night in Room 232 of the South Portland Comfort Inn. It was a nonsmoking room with two beds, an empty refrigerator, and a view of the parking lot. The hotel is a five-minute ride to the Portland Jetport, and just a few hundred yards from a barracks of the Maine State Police.

It is unclear why the pair went to Portland, although several investigators have speculated that it might have been to avoid having all 10 men walk into Logan en masse for the two flights they intended to hijack. Portland also is the closest airport with regular flights to Boston.

The other hijackers split into at least three other groups. Waleed M. and Wail Alshehri rented Room 432 at the Park Inn in Chestnut Hill. The guest in the next room, a writer from Nantucket named Michael Arnold, described them as ''just guys'' who did nothing unusual.

Two other hijackers, Ahmed Al-Ghamdi and Hamza Al-Ghamdi paid cash for a room at the Days Hotel, formerly the Days Inn, on Soldiers Field Road in Brighton. The hotel manager, Adam Sperling, said the FBI had collected a copy of the guest list, but declined further comment. Of the remaining four, two spent at least two nights, Sept. 9 and 10, in a room at the Milner Hotel on Charles Street in downtown Boston. A federal official said investigators are trying to determine whether the last two terrorists stayed at yet another Boston hotel on the eve of the hijackings, since they know of a related phone call placed from there.

Parking-lot fight, then deadly flight

On Sept. 11, Atta and Alomari were seen at 5:45 a.m. on a security camera at the Portland airport, heading toward US Airways Express Flight 5930, a 50-minute flight that was scheduled to leave at 6 a.m. It was slightly delayed, and they arrived at Logan International Airport just in time to board American Flight 11 before its 7:59 a.m. departure. Forty-nine minutes later, Flight 11 plowed into the north tower of the World Trade Center.

Three other hijackers drove the Mitsubishi to Logan on the morning of the attacks, authorities said. Investigators were alerted to the car after the attack when an unidentified man called police and said he had gotten into a screaming match with three agitated Arab men over an airport parking space that morning.

When investigators seized the Mitsubishi, they found materials including flight training manuals for Boeing 757 and 767 aircraft, and a copy of the Koran. A second rental car that some of the hijackers used also probably entered the Logan garage that morning, according to Massport records. It, too, was towed from a Logan garage after the attacks.

It was not immediately clear which of the hijackers were in the Mitsubishi, but all 10 made their flights. Fifteen minutes after Flight 11 hit the north tower, Flight 175 ripped into the south tower. The 10 hijackers had committed suicide. In the process, they murdered thousands inside the twin towers and hundreds of firefighters, police, and rescue workers who had tried to help.

The hijackers left few clues from their brief stays in Boston. Investigators have stripped the hotel rooms where they stayed, have questioned possible supporters among Boston cab drivers, and have interviewed employees of a Medford market. Sources said employees at the Wild Oats food store believe one of the terrorists purchased a large number of Massachusetts Lottery tickets on Sept. 9. FBI agents have questioned store employees about the incident and made plans to collect the videotape from a store surveillance camera.

However, Atta did leave one piece of evidence behind, one that spoke to his meticulous planning: In a piece of luggage that accompanied him from Portland but wasn't put on Flight 11, authorities found pilots' uniforms and a video of commercial aircraft. They also found what they first thought was as suicide note. It turned out to be Atta's will.

Rezendes reported from Florida; Zuckoff from Boston. They can be reached at rezendes@globe.com and zuckoff@globe.com.

Stephen Kurkjian, Sasha Pfeiffer, Shelley Murphy, Walter V. Robinson, Doug Belkin, and Bill Dedman of the Globe Staff, and Fran Riley and Arnold Markowitz, Globe correspondents, contributed to this report.

This story ran on page A1 of the Boston Globe on 9/23/2001. © Copyright 2001 Globe Newspaper Company.