Authorities said they received no unusual communications from American Flight 11, which left Boston at 7:59 a.m. with 92 people aboard or from United Flight 175, which departed 15 minutes later, with 65 people. Both were bound for Los Angeles, but both 767s were apparently hijacked by terrorists with knives and deliberately crashed into the twin towers in downtown Manhattan.

"Everything seemed normal when they left Logan," said Joseph Lawless, public safety director of the Massachusetts Port Authority. "We don't know how the hijackers accomplished what they did."

Logan Airport was evacuated and was to remain closed until further notice.

Lawless said the two jets that left Boston were believed to be the two that crashed in New York. Also Tuesday, an American Airlines jetliner that took off from Dulles International Airport crashed into the Pentagon and a plane from Newark International Airport crashed in Pennsylvania.

"It may be this was the most clever plot in the world and there was no way to stop it, but the starting assumption should be something is badly awry and we're going to go over every step," said Philip Heymann, a professor at Harvard Law School and an expert on terrorism.

Rep. Richard Neal, D-Mass., emerging from an evening briefing for lawmakers at Capitol police headquarters, said he thought the state's congressional delegation would converge in the next few days around the issue of how the hijacking could have happened at Logan.

"This was an inside job," said Neal. "We should be mindful of that in and around Logan."

Sen. Edward Kennedy, D-Mass., has started probing the question of how two airplanes could have been hijacked from Logan Airport, a source close to the senator said. Kennedy spent much of Tuesday on the telephone, including with Deputy FBI Director Thomas Pickard.

Planes waiting for takeoff at Logan were returned to the gates as air operations ceased nationwide. As the news spread, stunned passengers were first moved beyond security checkpoints and then evacuated entirely from the airport.

"There was silence," said Todd Hicks, who had hoped to return home to Eaton, Colo., after visiting his fiance in Cambridge. "I think people are dumbfounded that it could happen with all the security in place. We take our security for granted."

More than 150 state police detectives joined FBI and other federal investigators at Logan, said State Police Lt. Paul Maloney. He and other officials refused to discuss details of the investigation.

Massport officials said they planned security measures at least as stringent as those last implemented during the Persian Gulf War, including allowing only passengers past security checkpoints and eliminating curbside check-ins.

"One could speculate ... why we were chosen was because of our proximity to the New York area and the fact that we have wide-bodied aircraft leaving our airports fully loaded with fuel that participated in this tragic kamikazee-type attack on New York's World Trade Center this morning," said Massport Director of Aviation Thomas Kinton.

Kinton declined to comment on reports that terrorists had used knives or razors to hijack the aircraft.

Lawless said Globe Aviation Services Corp. operates security checkpoints for American Airlines at Logan. Danielle Crosby, the human resources manager for the Boston office, refused to comment, referring calls to the company's headquarters in Irving, Texas. A woman who answered the telephone at the headquarters also refused to comment.

Lawless said Huntleigh USA Corp. provides checkpoints for United Airlines. No one answered the telephone at the company's Logan office Tuesday. A woman who answered the telephone at the company's corporate headquarters in St. Louis, Mo., refused to comment.

Lawless declined to comment when asked if someone could have reached the planes from outside the terminals.

"Anybody involved is being questioned," Kinton said. "(It's) a massive investigation and we're cooperating fully with authorities to get them access to any personnel who may have been involved in working this morning."

In 1999, the major airlines at Logan and Massport were fined a total of $178,000 for at least 136 security violations over the previous two years, though an air travelers group said at the time that the violations were likely typical of major airports.

In the majority of incidents, screeners hired by the airlines to staff checkpoints in terminals routinely failed to detect test items, such as pipe bombs and guns.

Also in 1999, a teen-ager who said he wanted to impress the Israeli intelligence agency allegedly sliced through a fence and settled into an empty seat on a British Airways jet and flew to London.

"We have a very high security standard here," Lawless said Tuesday. "We consider ourselves as secure, if not more secure, than any other airport in the United States."

Numerous experts said only a police state could implement truly impenetrable security.

"It shows that given the democratic society we live in it's next to impossible to catch a truly diabolical yet brilliant attack," said Richard Gritta, an expert on airport security at the University of Portland, Ore..

Charles Arena, head of public safety at Massport from 1979 to 1988, said he believed the airport is as safe as any but Boston, but its cosmopolitan population makes it difficult to profile potential terrorists.

"I think we should start taking a good hard look at the way the Israelis and some other Europeans handle security at their airports," he said. "People don't want to be inconvenienced, but I'll take a little inconvenience over what's happened today."

"The terrorists are trying to undermine the confidence of the average U.S. citizen in the safety of U.S. transportation," Gritta said. "And I think they've done a pretty good job."