Chilling radio transmissions by the Sept. 11 hijackers from the planes they commandeered were played publicly for the first time Thursday, providing a vivid and horrifying window into the events that unfolded during the worst terrorism attack in U.S. history.

"We have some planes. Just stay quiet and you'll be O.K. We are returning to the airport," a hijacker, believed to be Mohamed Atta (search), the alleged ringleader of the 19 hijackers, told the passengers of American Airlines Flight 11 (search). The tape was played at the Sept. 11 commission's final public hearing, which was attended by some of the relatives of the 9/11 victims.

Click to read the Sept. 11 Commission's report (pdf).

That transmission was the first inkling federal air traffic controllers had of the hijacking of Flight 11 shortly after takeoff from Boston's Logan Airport at 8 a.m. EDT. Atta had been speaking to the plane's passengers, but the radio transmission was received at the FAA's Boston center.

As FAA controllers tried desperately to contact the plane, they picked up another transmission, also apparently from Atta.

"Nobody move. Everything will be O.K. If you try to make any moves, you'll endanger yourself and the airplane. Just stay quiet."

Controllers tried to contact the military, even trying to raise an alert center in Atlantic City, N.J., unaware that facility had been phased out. The FAA finally reached the appropriate military office at 8:37 a.m.

"We have a problem here," the FAA's Boston center told NEADS, the North East Air Defense Sector (search). "We have a hijacked aircraft headed towards New York, and we need you guys to, we need someone to scramble some F-16s or something up there, help us out."

"Is this real-world or exercise?" asked the NEADS officer.

"No, this is not an exercise, not a test," the FAA responded.

F-15 fighter jets were ordered scrambled at 8:46 a.m. But 40 seconds later, Flight 11 hit the north tower of the World Trade Center.

For United Flight 175, the second plane hijacked from Logan, the situation was similarly disjointed. That plane took off at 8:14 a.m. At almost the same time as Flight 11 crashed into the World Trade Center, Flight 175 changed the code for its transponder, the mechanism planes to use identity themselves to controllers.

At 8:58 a.m., a controller at the FAA's New York center told another New York controller, "We might have a hijack over here, two of them." At 9 a.m., a New York center manager told the FAA Command Center in Herndon, Va., "We have several situations going on here. It's escalating big, big time. We need to get the military involved with us."

Flight 175 hit the south tower of the World Trade Center at 9:03 a.m.

The third hijacked plane, American Airlines Flight 77, left Dulles International Airport (search) near Washington at 8:20 a.m. At 8:54 a.m., the plane deviated from its flight plan. It was tracked by an Indianapolis-based controller, who at the time was unaware of the other hijackings. When the controller couldn't raise the aircraft, it notified other agencies that it was missing and may have crashed.

The military did not know about the search for Flight 77. Instead, it was mistakenly told by the FAA's Boston center that Flight 11 still was in the air and headed toward Washington. Fighter jets were ordered scrambled from Virginia's Langley Air Force Base (search) at 9:24 a.m.

But instead of heading north to Washington, the flew east because the initial scramble order didn't include the target's location or distance. A "generic" flight plan incorrectly led the fighter jet pilots to believe they were to fly east for 60 miles, the report said.

FAA radar tracked Flight 77, but for what the commission said were technical reasons, the information was not immediately displayed to controllers at the Indianapolis center. It eventually re-emerged on radar, and by 9:32 a.m. controllers at Dulles observed that it was headed toward Washington.

The FAA asked an unarmed military cargo plane to identify and follow the airliner. At 9:38 a.m., the pilot of that plane reported to the Washington control tower that it "looks like that aircraft crashed into the Pentagon, sir."

The fourth plane, United Airlines Flight 93, took off from Newark, N.J., at 8:42 a.m. Its last transmission was at 9:28 a.m. A minute later, the Cleveland-based FAA controller heard "a radio transmission of unintelligible sounds of possible screaming or a struggle from an unknown origin."

There was a second transmission from the Flight 93 cockpit, with sounds of screaming and someone yelling, "Get out of here, get out of here." Then came another transmission. "Keep remaining sitting. We have a bomb on board."

Between 9:34 a.m. and 9:38 a.m., the controller observed Flight 93 climbing and moved several aircraft out of its way. Then another transmission came from the plane.

"Uh, is the captain. Would like you all to remain seated. There is a bomb on board and are going back to the airport, and to have our demands [unintelligible]. Please remain quiet."

Flight 93 was spotted by another aircraft and reported to be "waving its wings," radical gyrations the commission believes the hijackers employed to try to defeat the passenger uprising. The plane crashed at 10:03 a.m. in a Pennsylvania field 125 miles from Washington.