The title is Loose Change." It may be the internet's first blockbuster movie. It says: "If the government has nothing to hide, why are they so afraid to answer a few questions?" Ten million have watched so far, and that's only a fraction of the 42 percent of Americans recently polled by Reuters who believe its message -- that our own government covered up critical evidence of 9/11. But it's more than conspiracy theories on the Internet at work here. Some of the very people who first investigated 9/11 admit there's still a lot they don't know about the plot. Dale Watson ran counterterrorism for the FBI and led the investigation into 9/11. And one of the things that still puzzles him most is the Las Vegas connection. All four of the 9/11 hijacker pilots flew Las Vegas in the months preceding the attack. Some came twice. But once here, the men essentially disappeared. On his first trip here hijacker Muhammad Atta rented a car, and drove it 110 miles in two days. But where did he go? Who did he visit with, if anyone? There's not a clue. Why did Muhammad Atta and his sidekick travel all the way to Portland, Maine, to connect with -- and almost miss -- the Boston flight they later hijacked? Why did Atta earlier travel all the way from Florida to Virginia Beach, Va., just to rent a mailbox? The FBI still doesn't know -- but hasn't stopped looking. Five years after the crime a dozen agents remain on the 9/11 case. And they still get 20 new leads a week.