The hijacking of United Airlines Flight 93 on Sept. 11 began with screams and sounds of a struggle, followed by the breathless but eerily commanding voice of a terrorist trying to exert control over the passengers on the doomed airliner, according to newly obtained tape recordings from the cockpit.

Aired publicly for the first time Thursday night by ABC News's "PrimeTime" newsmagazine, the recordings help fill in some of the blanks in the story of the Boeing 757's last minutes before it crashed in rural Pennsylvania. An apparent revolt by some passengers prevented the hijackers from striking an unknown target in Washington.

However, the tapes do not include any sounds from the aircraft's final moments, when passengers are believed to have stormed the cockpit, precipitating its downfall. The plane's cockpit voice recorder, which presumably would include that information, was recovered from the crash site near Shanksville, Pa., but is still being analyzed by federal investigators.

About 9:30 a.m. on Sept. 11, a United Airlines pilot speaking into an open microphone in the cockpit can be heard calmly asking a Cleveland-based air traffic controller for more information about the crisis unfolding in New York.

Less than a minute later, the same microphone captured a cacophony of screaming and yelling as the hijackers began to take over the plane. Passenger accounts via GTE Airfones and personal cell phones from aboard the plane have indicated that two men invaded the cockpit.

Thirty seconds later, there are more screams, followed by an American crew member's agonized voice, saying: "Get out of here. Get out of here."

The brief gap between what seem to be two separate and violent moments raises the possibility that those words could have been directed either to one of the hijackers who initially seized the cockpit or to another person inside the plane who may have rushed forward to repond to the struggle.

This sequence makes plain that both men at the plane's controls that day -- Captain Jason Dahl, 43, and First Officer Leroy W. Homer, 36 -- were physically subdued by the hijackers, but the excerpts broadcast on the ABC program did not indicate whether they were injured or killed. In cell phone calls, passengers said the hijackers were wielding knives.

Passengers also spoke of seeing two people lying motionless on the floor near the cockpit, possibly with their throats cut.

With the pilots subdued, there is only silence from the cockpit as the Cleveland air traffic controller tries to reestablish contact with the plane.

Then a hijacker, out of breath from the apparent struggle and seemingly unaware that his words are still audible to controllers on the open microphone, tells passengers and remaining flight attendants: "Ladies and gentlemen, it's the captain, please sit down." The accented voice may have been that of Ziad Jarrahi, identified as one of the terrorists. "Keep remaining sitting. We have a bomb aboard."

Apparently that order was not sufficient. "Hi, this is the captain," the hijacker at the controls says a bit later. "We'd like you all to remain seated. There is a bomb aboard. And we are going to return to the airport. And they have our demands, so please remain quiet."

Shortly thereafter, the hijackers shut off the open microphone.