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 left 15 minutes later, with 65 people. The 767s were both bound for Los Angeles.

``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.''

The Boston Herald, quoting a source it did not identify, reported that authorities had seized a car at Logan airport that contained Arabic-language flight training manuals. The source said five Arab men had been identified as suspects, including a trained pilot. At least two of those men flew to Logan on Tuesday from Portland, Maine, the Herald said.

The luggage of one of the men who flew to the airport Tuesday didn't make his scheduled connection. The Boston Globe reported the luggage contained a copy of the Koran, an instructional video on flying commercial airliners and a fuel consumption calculator.

The FBI refused to comment on the reports.

The airport was evacuated and shut down until further notice.

Also Tuesday, an American Airlines jetliner that took off from Dulles International Airport near Washington crashed into the Pentagon and a plane from Newark International Airport crashed in Pennsylvania.

Airport security at Dulles and Newark ó and at Reagan National Airport near Washington ó were also under scrutiny.

FBI spokeswoman Sandra Carroll declined comment on the Newark investigation, as did Federal Aviation Administration officials on airport security at Dulles.

``We're not going to speculate or comment on security until we figure this out,'' said FAA spokeswoman Arlene Salac.

In Boston, more than 150 state police detectives joined FBI and other federal investigators at the airport, State Police Lt. Paul Maloney said.

Port authority 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 kamikaze-type attack on New York's World Trade Center this morning,'' Port authority aviation director Thomas Kinton said.

Lawless said Globe Aviation Services Corp. of Irving, Texas, and Huntleigh USA Corp. of St. Louis operate security checkpoints for the two airlines at Logan. People at both companies' headquarters refused to comment.

In 1999, the major airlines at Logan and the port authority were fined a total of $178,000 for at least 136 security violations over the previous two years. 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.''