Washington ? Chilling radio transmissions by the Sept. 11 hijackers from the planes they commandeered were played for the first time Thursday, providing a vivid and horrifying portrait as they unfolded on that fateful day before confused air traffic controllers and military personnel.

“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, the alleged ringleader of the 19 hijackers, told the passengers of American Airlines Flight 11. The tape was played for the audience at the commission’s hearing.

That transmission was the first inkling federal air traffic controllers had of the hijacking of American Airlines Flight 11 shortly after takeoff from Boston’s Logan Airport at 8 a.m.

As FAA controllers tried desperately to contact the plane, which had changed its transponder code, 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 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. “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 incredulous NEADS officer.

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

F-15 fighter jets were ordered scrambled from Otis Air Force Base at 8:46 a.m. Forty 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. from Boston’s Logan Airport. At 8:47 a.m., almost the same time as Flight 11 crashed into the World Trade Center, Flight 175 changed its transponder code.

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

The third hijacked plane, American Airlines Flight 77, had left Dulles International Airport near Washington at 8:20 a.m. At 8:54 a.m., the plane deviated from its flight plan.

The military was mistakenly told by the FAA’s Boston Center that American’s Flight 11 was still in the air and headed toward Washington. Fighter jets were ordered scrambled from Langley Air Force Base at 9:24 a.m. and headed toward Washington.

Flight 77 eventually re-emerged on radar, and by 9:32 a.m. controllers observed that it was headed to 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 control tower that it “looks like that aircraft crashed into the Pentagon, sir.”

United Airlines Flight 93 had taken off from Newark 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, with sounds of screaming 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 United 93 climbing and moved several aircraft out of its way.

United 93 crashed at 10:03 a.m. near Johnstown, Pa.