The data recorder may yield clues about those passengers' actions, but investigators were still searching for the jet's voice recorder that could reveal human details of an onboard conflict.
The mystery of Flight 93 took a startling twist early Thursday when the FBI refused to rule out the possibility that the jet had been shot down. But later, special agent Bill Crowley said unequivocally, "There was no military involvement in the plane going down."
The Defense Department has vigorously denied suggestions that the military could have downed the jet to spare a target in the nation's capital.
Local residents said they had seen a second plane in the area, possibly an F-16 fighter, and burning debris falling from the sky. Crowley said investigators had determined that two other planes were nearby but didn't know if either was military.
Crowley said National Transportation Safety Board experts believe "it is probable" debris could have scattered as far as 8 miles. The material was lightweight — paper, nylon webbing and "things that if they were in the air would easily blow," he said. The wind at the time on the ground was blowing about 10 mph.
But debris found that far from the crash site raises other possible scenarios, experts say. If the plane was flying in high-stress maneuvers, perhaps caused by a cockpit struggle, pieces could have come off. A door in the fuselage could have been opened, perhaps by passengers who already knew other planes had hit the World Trade Center towers and wanted to sabotage their own flight to save lives on the ground. Another plane could have fired on the United jet. Or the terrorists' alleged bomb, which they claimed was inside a red box, could have gone off.
Investigators stressed that, this early in the investigation, little has been ruled out. Kevin Darcy, an accident investigation and safety consultant in Seattle, says without more evidence, it's hard to speculate on debris scenarios. But each has its problems, he says.
Parts rarely come loose from airborne jets, even under extreme conditions, Darcy says. The Boeing 757, in particular, is a robust plane and would have had to be going "very, very fast to get parts shedding," he says.
Doors on modern jets are virtually impossible to open once the cabin is pressurized. But if the cabin had lost pressure, perhaps from a hole caused by a bomb, debris could have flown out. A Pittsburgh TV station reported that the charred pages from United's in-flight magazine and what appeared to be financial documents from a New York brokerage house were found at least 6 miles from the crash.
Darcy says if the plane had been shot down "you might expect to see a lot more stuff come down" ahead of the jet's final plunge. But, he says, "I don't see anything yet that would rule that out. The real keys will be to understand the final path of the plane and the winds." Strong winds on the ground could have blown papers and even light wreckage debris, such as insulation, a great distance, Darcy says.
The San Francisco-bound plane left Newark, N.J., about 8 a.m. ET, but before it reached Cleveland abruptly turned back east, lost altitude and flew erratically across Pennsylvania toward Washington.
Based on phone calls during the frantic final minutes of Flight 93, relatives of three male passengers said they think their loved ones fought back and tried to help ditch the plane.
"There was no doubt that he was forming a plan to get the hijackers out of the cockpit and get home that night," Deena Burnett said of her husband, Thomas. "He was not reviewing his life. He was not telling me wonderful things. He's a take-charge kind of guy."
Burnett called his wife four times. The last time, he said: "We're getting ready to do something.
CNN reported that on a partial transcript of talk from the plane recorded by air traffic controllers someone shouted, "Get out of here." Later, amid sounds of scuffling, someone yelled again, "Get out of here."
Then a voice said, "There is a bomb on board. This is the captain speaking. Remain in your seat. There is a bomb on board. Stay quiet. We are meeting their demands. We are returning to the airport." CNN said a source who had heard the tape said the voice spoke in broken English. United had no comment on the transcript.
The passengers who from their calls convinced relatives they were ready to take matters into their own hands were Burnett, 38, of San Ramon, Calif., Jeremy Glick, 31, of West Milford, N.J., and Mark Bingham, 31, of San Francisco.
"Jeremy and the people around them found out about the flights into the World Trade Center and decided that if their fate was to die, they should fight," Glick's uncle, Tom Crowley said.
Bingham's mother, Alice Hoglan, said it would not have been at all out of character for her 6-foot-5, 230-pound son to go after the hijackers. He had run with the bulls in Pamplona, Spain, last summer and wrestled a gun away from a mugger several years ago.
"He doesn't seek out trouble, but he won't run away from it, either," Hoglan said. "If he sees something wrong, he sets it right."
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