WASHINGTON (CNN) -- FBI analysis of the cockpit voice recorder from United Flight 93 indicates one of the hijackers may have instructed another hijacker to crash the plane.
Officials, however, stressed there is no leading theory among investigators about how the plane went down.
According to a report sent to Congress last month on the September 11, 2001 terror attacks, FBI Director Robert Mueller told the congressional 9-11 committee that bureau analysts thoroughly examined the cockpit tape and that one hijacker, minutes before the plane hit the ground, "advised (hijacker) Ziad Jarrah to crash the plane and end the passengers attempt to retake the airplane."
Flight 93 crashed in a Pennsylvania field on that day, with 33 passengers, seven crew members and four terrorist hijackers aboard. The passengers have long been hailed as heroes for charging the hijackers in an effort to prevent them from crashing the plane into a landmark building, a charge authorities have said began with the rallying cry, "Let's roll."
FBI officials said they will never know for sure what happened in the final moments of the flight. The cockpit voice recorder contains a lot of static and yelling in both English and Arabic, as well as other noises, including the sound of breaking glass, officials said.
They said the tape was enhanced and analyzed by pilots, engineers and other professionals.
After the crash of Flight 93 in Pennsylvania, several victims' family members recounted heart-wrenching stories about last-minute phone calls to them right before the plane crashed, in which people could hear what sounded like struggles going on between passengers and some of the hijackers. Another theory has been that some of the passengers were able to break into the cockpit and force the plane down.
FBI spokesman Susan Whitson told CNN there is no conclusion regarding exactly what happened on Flight 93.
"The only thing we can say definitively is that the passengers aboard that flight were heroes," Whitson told CNN.
Another mystery surrounding Flight 93 is its intended target. Officials believe the plane's hijackers were trying to force the plane to Washington, D.C. The plane made a sudden turn over Ohio after it left Newark, New Jersey, en route toward San Francisco. While some officials have speculated the target was the U.S. Capitol, others believe it was the White House.
Asked last Sunday whether the Capitol was the intended target on CNN's "Late Edition with Wolf Blitzer", Homeland Security Secretary Tom Ridge said, "There was some dispute as to whether it was the White House or U.S. Capitol. And I don't think we'll ever know."