Two jets that left Boston's Logan International Airport this morning - American Airlines Flight 11, with 92 people aboard, and United Airlines Flight 175, carrying 65 people - were hijacked, and airline officials said both planes had crashed.

United officials have yet to disclose where Flight 175, a Boeing 767 that departed at 7:45 a.m., went down. The American Airlines flight was one of two aircraft that slammed into New York's World Trade Center towers, both of which later collapsed into rubble.

On board American's Flight 11 were 81 passengers, nine flight attendants, and two pilots, said Laura Mayo, a spokeswoman for American Airlines. All are believed to be dead. Mayo said that because of this morning's events - the flight was one of two American jets that crashed today - the airline has been advised by the federal government not to release details yet.

However, Donald J. Carty, American's chief executive, issued a statement that said: "We are horrified by the tragic events. Our thoughts and prayers go out to the families of all involved."

American Airlines Flight 11 left at 7:59 a.m., on time for a nonstop flight to Los Angeles, according to a Massport official.

There were no reports that anything unusual happened in or around the jet, which Massport officials believe was highjacked by terrorists, prior to takeoff at 7:45 a.m.

One airport employee, who asked not to be identified, said the American flight left on time from Gate 32 in Terminal B, and that nothing unusual was apparent. The employee said airline workers learned almost simultaneously that there had been explosions at the World Trade Center and that air traffic control had lost contact with the American flight.

At least one flight that left Logan this morning encountered heightened security precautions upon landing elsewhere. A flight that arrived in Cleveland was being searched on the ground in a secure area, a spokesman for Mayor Michael White of Cleveland told Reuters. "There was a plane that landed inbound from Boston that was cordoned into a secure area and is being investigated," the spokesman said.

At Logan, officials closed and sealed off the aiport early this afternoon. People arriving to get information about those passengers on the flights that crashed were being put into black limousines and driven to the Hilton Hotel at the airport.

State Police were sweeping the airport with bomb-sniffing dogs, according to Trooper Joe Machado. Steel security grates were shut down at 10 a.m. at Terminal B. People were allowed out, but no one was allowed in.

Inside the airport it, was eerily quiet at 10:30 a.m. as the entire facility headed into lockdown. An announcement made over the public address system said that some baggage was being held from some flights. Passengers were asked to show identification before they picked up their bags from flights that had arrived.

Long lines formed at pay phones as passengers tried to call their families, and the Sam Adams Bar in Terminal C was packed with grounded flight crews watching the news unfold on television.

A Massport shuttle bus driver started crying on his rounds. "These people, I laugh and joke with them every morning," said Mike Walen, a 14-year veteran.

Alice Price, 62, was at Logan on her way to Oklahoma City, where she lives. Price said two of friends had died in the Oklahoma City bombing.

"I'm terrified," she said. "We've been in tears. It definitely brings it back."

Timothy Malloy, 31, an investment analyst from Natick on his way to London for a business trip, said his plane was on the runway, ready to take off, when it was brought back. From a pay phone, he called family members who thought he was on the plane that crashed. Malloy and his wife are expecting a baby. "I'm going to cancel my trip," he said.

Bill Shields was on a flight from Logan to San Diego when the pilot announced they would not be leaving, stunning the packed jet. "It was almost surreal," Shields said. "No one said a word."

Shields, who recently accepted a job that would involve much air travel, immediately called his wife, who was in tears, and then his parents.

Their message, he said, was simple. "They said: `I don't care what you do. You can pick up garbage. But don't fly.' "