Eye witness accounts mainly.
From a thread: "Have you not heard the Haskell's testimony about how a "sharp dressed man" got the underwear bomber on the plane? The "bomber" had no passport but this man talked with some airport supervisor and next thing you know he was on the plane! Also, once on the plane other witnesses saw another man take out his video camera and tape the... "terrorist" from the beginning of the flight all the way until he tried to blow up the plane and his detainment. No word on that guy either. Then we have another person that was arrested off the flight after they landed when a bomb sniffing bomb alerted authorities to something suspicious in his carry on luggage. This guy was lead away, questioned and then dragged off in handcuffs! The FBI has changed their story at least five times now while the Haskell's story has been consistent from day one. Yes, there are actually real Muslim terrorists but this one clearly was not. He was some dupe playing part in some drill to get the public behind another war in Yemen, the now new "safe haven" for terrorists."
This is first part of an interview with one eye witness,
The other two parts can be found on youtube "Bombshell Eyewitness Revelations: Confirmed FBI Cover-Up Of Flight 253 Attack".

I hope the video works, it works in wimpy flv but not in mpc.