Five years after 9/11, it's the question that brings the most frightening perspective to U.S. foreign policies today: Did the governments of Saudi Arabia or Pakistan, now strong U.S. allies, have advance knowledge of the 9-11 attacks?
"There's a strong likelihood that individuals within Saudi Arabia and Pakistan were well aware of the plot being carried out by Mohammed Atta and company,” said NBC terrorism analyst Steve Emerson. “It doesn't necessary mean the regimes were aware, but certainly there was knowledge about a Fatwa, or religious decree, mandating that Muslims crash planes into buildings."
Before 9/11, the Pakistani government was one of only three in the world to recognize and have diplomatic relations with Afghanistan's al-Qaida-dominated Taliban regime, and Pakistani intelligence officials had deep contacts.
"Their service, called the ISI, had a very strong relationship with the Taliban,” NBC's Roger Cressey said. “They clearly had links inside Al-Qaeda. So the question is did the ISI have prior knowledge of this plot? And if they did, did they inform Islamabad and President Musharraf?”
When it comes to Saudi Arabia, 15 of the 19 hijackers were Saudi nationals. Some of them received money in the U.S. through an account that went through the Saudi embassy.
"We still don't know if the Saudi government had prior knowledge of the attacks,” Cressey said. “What it does mean, though, is that the Saudis did have a relationship with the hijackers.”
Some of the most intriguing information about any possible advance knowledge in Pakistan or Saudi Arabia has come through information from captured al-Qaida terrorists.
Six months after 9/11, U.S. and Pakistani forces successfully abducted al-Qaida leader Abu Zubayda. President Bush said Zubayda was, "spending a lot of time as one of the top operating officials of Al-Qaeda, plotting and planning murder."
Under interrogation, as first reported by journalist Gerald Posner, Zubayda gave personal contact information about four Saudi Princes and the head of Pakistan's air force. Zubayda charged the men were deeply involved with Osama bin Laden and had been meeting with bin Laden for years.
Officials from Pakistan and Saudi Arabia denied the charge, and U.S. officials say Zubayda later recanted, saying he lied to avoid torture.
But in the summer of 2002, shortly after the CIA shared Zubayda's initial claims with Saudi and Pakistani intelligence officials, four of the five men named by Zubayda died within weeks, each under strange circumstances.
Were the officials murdered so the Saudi and Pakistani governments could bury embarrassing pre-9/11 links to Al-Qaeda? U.S. intelligence analysts aren't sure.
In the early 1990's, the Saudi royal family took a "see no evil" approach to bin Laden. Then, after bin Laden vowed to destroy the Saudi regime, the government revoked bin Laden's citizenship in 1994. By the late 1990's, the Saudi royal family considered bin Laden an enemy.
Still, three years ago when Congress released a report on 9/11, 27 pages were redacted.
President Bush said that "declassification of that part of a 900 page document would reveal sources and methods that will make it harder for us to win the war on terror. But intelligence sources in Congress say the redacted section contains information about Saudi Arabia.
Did the Saudi government know about 9-11 in advance?
Michael Scheuer is a former CIA analyst.
"Saudi Arabia said they were expecting a big operation, but they didn't know when it was going to happen exactly,” he said. “So, while I don't think we've documented any assistance or foreknowledge, I think it would be whistling past the graveyard to assume people didn't know that something bigger was coming than we had seen before.”
As for Pakistan, U.S. analysts believe Pakistani intelligence had more information about 9/11 than President Musharraf has acknowledged.
"I think the Pakistani intelligence service did know something was going on,” Cressey said. “I think they probably did know a big attack might be coming. I'm not sure they had the tactical information of where, when, and how. At least I hope they didn't.”