On Tuesday, hijackers crashed two passenger jets into the World Trade Center in New York City, toppling the 110-story twin towers and killing all aboard the jets and an unknown number on the ground and in the building.
In a similar attack, another passenger plane struck the Pentagon, leaving a gaping gash in the nation's military hub, killing all on board the plane and more on the ground. Within an hour, a fourth airliner went down near Pittsburgh, killing all aboard.
According to the FBI, the planes involved in the trade center crashes were American Airlines Flight 11, a Boeing 767 en route from Boston to Los Angeles carrying 81 passengers, nine flight attendants and two pilots, and United Airlines Flight 175, a Boeing 767 also headed from Boston to Los Angeles. Officials said the plane was carrying 56 passengers, two pilots and seven flight attendants.
On Tuesday night, FBI issued several search warrants on U.S. residents based on preliminary information on how two of the planes were hijacked. They said a flight attendant called authorities from the airborne American Airlines Flight 11, reporting that two flight attendants had been stabbed and that intruders had broken into the cockpit. The Christian Science Monitor reported that air traffic controllers could hear the hijackers of Flight 11 instructing the pilots in English.
A passenger on the United flight that crashed outside Pittsburgh called authorities from his cell phone, saying the plane had been hijacked. A senior FBI official told ABCNEWS that investigators received reports that other passengers were able to call 911 or relatives, and described horrific scenes of terrorists slashing throats.