The Pennsylvania Crash: What Happened?

N E W   Y O R K, Sept. 12 -- Investigators are trying to reconstruct what happened to United Airlines Flight 93, which crashed in western Pennsylvania — the only one of the four planes hijacked on Tuesday that was not steered into a famous, densely inhabited building.

Phone calls from passengers, made in the frantic minutes after the plane was hijacked, yield scant but potentially revealing clues about a possible on-board struggle that may have led to the crash — but prevented hijackers from taking the controls.

And since Flight 93, unlike the other three planes that were hijacked, did not cause an inferno-like fire by crashing into a building, it is possible that some information from the on-board flight recorders could be recovered — which seems less likely in the other cases.

Law enforcement officials and aviation authorities do not know yet why the plane, a Boeing 757-200 bound from Newark, N.J., to San Francisco, crashed in Pennsylvania's rural Somerset County, about 80 miles southeast of Pittsburgh.

All seven crew members and 38 passengers on board were killed.

U-Turn and Request for New Flight Plan

Federal Aviation Administration data shows Flight 93, which left Newark at 8:01 a.m. ET, followed its normal flight plan until it neared Cleveland, where the plane took a hard turn south.

That marks the point at which the plane must have been hijacked, investigators say. It then took a turn to the east.

ABCNEWS has learned that shortly before the plane changed directions, someone in the cockpit radioed in and asked the FAA for a new flight plan, with a final destination of Washington.

Passengers Make Desperate Phone Calls

At this stage, passengers realized what had happened, since several of them called from phones on board to report the hijack.

"We are being hijacked, we are being hijacked!" one man on board said during a brief cell phone call from a plane restroom to a 911 dispatcher.

The emergency dispatcher, Glenn Cramer, told The Associated Press the man on board reported the plane "was going down. He heard some sort of explosion and saw white smoke coming from the plane and we lost contact with him."