One reason for the limited understanding is that it was only after September 11 that the US government began to aggressively investigate the issue. Prior to September, the FBI apparently did not focus investigative resources on [REDACTED]
Saudi nationals in the United Slates due to Saudi Arabias status as an American ally.'
Analysis: An intelligence organization is tasked with investigating all foreign presence and activities as related to the home country. It would be unusual for an intelligence body to simply avoid investigating an ally. The surveillance, information gathering, etc., might be limited and the methods might be different, but all foreign governments with a presence in the United States are investigated. Most every embassy has an intelligence operation (see background above). At the very least, those players would be monitored to some extent.
So what is the real reason that the FBI and the CIA both managed to avoid looking into Saudi Arabia and its relationship to terrorism? What we have learned in the 14 years since this document was classified is that they were told not to investigate the Saudis by consecutive Presidents of the US.
Context: What we do know is that the FBI was always a bit constrained when it came to investigating members of the bin Laden family. According to FBI sources, this policy of hands off the Saudis, became much worse and more broad after the Bush administration took office. The FBI was told to back off investigating any of the Saudi royals as well as the bin Laden family.
The question is why? The answer may be related to the alleged deal that Prince Turki made with bin Laden (see background above). How likely is it, however, Saudi protection money would be paid to operatives in the US directly linked to the hijackers rather than paid through Middle Eastern channels and directly to bin Laden and the Taliban?