Jan. 15, 2000: Hijackers Khalid al-Mihdhar and Nawaf al-Hazmi land in Los Angeles — then disappear for two weeks.
After almost three years, neither the FBI nor the 9/11 commission knows what the two hijackers, who spoke very little English, were doing during that time.
Still, unanswered: did the hijackers have a support network in the U.S.?
"There's lots of smoke, there are lots of coincidences ... the plotters, 9/11 co-conspirators had help, we just don't know how much help and how witting or unwitting that was," says 9/11 commissioner Tim Roemer.
The 9/11 commission says "the evidence is thin" but "worrisome," especially involving three men who crossed paths with the hijackers in California.
Modhar Abdullah
He's a Yemeni student who befriends the two hijackers and helps them apply to flight schools. The commission says Abdullah knew one of the hijackers was involved with a group tied to al-Qaida and, after 9/11, "expressed hatred for the U.S. government." But in an exclusive phone interview with NBC News from Yemen, Abdullah denies it, and says the hijackers tricked him.
"They never even mentioned they had training before. They didn't mention they have such hatred to the United States," says Abdullah.
Abdullah repeatedly told the FBI he did not have prior knowledge of the attacks. But the commission says Abdullah bragged that he did know... in advance. Abdullah denies it.
Abdullah was deported to Yemen before the 9/11 commission finished its investigation. That's proof, critics say, the FBI still hasn't learned the lessons of 9/11.
"They end up not prosecuting him, not even keeping him for intelligence purposes," says commissioner Roemer.
Fahad al-Thumairy
He's a Saudi diplomat in Los Angeles and Imam of a mosque frequented by the hijackers. Thumairy was barred from the U.S. last year. U.S. officials say that's because of suspected links to al-Qaida. The 9/11 report calls Thumairy a "logical" contact for the hijackers, but finds no evidence he actually helped them and he denies it.
Anway Aulaqi
He's an Imam at a San Diego mosque frequented by the hijackers, and was investigated for ties to other suspected terrorists. Aulaqi admits meetings with a hijacker, but denies knowledge of the plot. Yet, after Aulaqi moved to a Virginia mosque, the report says, the hijackers showed up there.
"Is it worrisome? Is it suspicious? Are there clues that are just not answered? Absolutely," says Roemer.
The FBI still insists the hijackers had no accomplices in the U.S. But the 9/11 commission believes the FBI still hasn't adequately investigated whether a network of sympathizers secretly lent a hand.