What is known is: a Boeing 757, United Airlines Flight 93, took off at 8:42 AM from Newark, headed for San Francisco, turned off its transponders, heading back eastward, and, after losing an engine and flying 7 miles upside down, crashed in Shanksville, Pennsylvania at 10:06 AM. The FBI determined the identities used by four hijackers.

What investigators know about this hijacking comes from three sources.

First, at 9:28 AM, a ground controller heard someone speaking in broken English announce over the planes loudspeakers that there was a bomb on the plane and it was returning to the airport. Presumably, the plane had been hijacked at that point.

Second, at least 5 passengers and one flight attendant reported events over their cell phones while the hijacking was in progress. None described the attack on the pilots or cockpits. One witness reported that one hijacker had a "bomb strapped on." Two others reported hijackers armed with knives. One reported a gun.

Third, both the flight data recorder and cockpit voice recorder, which contained the last 30 minutes of sounds in the cockpit were recovered. The FBI also recovered a letter from one of the hijackers so presumably, if a letter survived the crash, other evidence was not incinerated. The FBI has kept secret: a) the inventory of what was found at the crash site; b) the results of the examination of the wreckage and autopsies of passengers (if they were performed), c) the data from the flight recorder d) the recording and transcript of the cockpit recorder. There is therefore a divergence between what the FBI may know and what has been disclosed to the public.

Here is what is unknown to the public:

1) What weapons did they hijackers have?

There are reports of a bomb, a gun and knives. What weapons were found at crash site?

2) How did these get these weapons through security at Newark Airport? Bomb, guns and knives should all be detected by scanners.

3) How did the hijackers gain entry to the cockpits?

None of the pilots had the opportunity to sound a May Day alert or scream. How did the hijackers get through the supposedly locked doors so as not to alert the pilots?

4) How did the hijackers immobilize the pilots without causing the planes to swerve out of control?

The pilots are strapped by harnesses into their seats and have microphones connecting them to ground controllers. How did the hijackers managed to get the pilots out of their seats and take over the controls? were the pilots' bodies found in the wreckage?

5) Were the planes on auto pilot? If so, how did the hijackers determine this prior to the attack?

6) What was the target of the plane? Were any global positioning devices found in the wreckage? If so, was the target keyed in?

7) What did the hijackers talk about during the last 30 minutes of the flight? Can the airline pilots be heard by enhancing the tape?

8) What caused the plane to crash?

What problems are indicated by the the flight recorder, cockpit recorder and other evidence in wreckage?