Apparent the FBI was supposed to monitor an overseas operation the involved Saudi nationals learning how to fly planes.
The CIA prevented the FBI from reporting their investigation to higher-ups.
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The CIA prevented the FBI from reporting their investigation to higher-ups.

Bombshell filing: 9/11 hijackers were CIA recruits ā NaturalNews.com
At least two 9/11 hijackers had been recruited into a joint CIA-Saudi intelligence operation that was covered up at the highest level, according to an explosive new court filing. (Article by Kit Klarenberg republished from TheGrayzone.com) A newly-released court filing raises grave questions...
āA 50/50 chanceā of Saudi involvement
In 1996, Alec Station was created under the watch of the CIA. The initiative was supposed to comprise a joint investigative effort with the FBI. However, FBI operatives assigned to the unit soon found they were prohibited from passing any information to the Bureauās head office without the CIAās authorization, and faced harsh penalties for doing so. Efforts to share information with the FBIās equivalent unit ā the I-49 squad based in New York ā were repeatedly blocked.
In late 1999, with āthe system blinking redā about an imminent large-scale Al Qaeda terror attack inside the US, the CIA and NSA were closely monitoring an āoperational cadreā within an Al Qaeda cell that included the Saudi nationals Nawaf al-Hazmi and Khalid al-Mihdhar. The pair would purportedly go on to hijack American Airlines Flight 77, which crashed into the Pentagon on 9/11.
Al-Hazmi and al-Midhar had attended an Al Qaeda summit that took place between January 5th and 8th 2000, in Kuala Lumpur, Malaysia. The meeting was secretly photographed and videotaped by local authorities at Alec Stationās request although, apparently, no audio was captured. En route, Mihdhar transited through Dubai, where CIA operatives broke into his hotel room and photocopied his passport. It showed that he possessed a multi-entry visa to the US.
A contemporaneous internal CIA cable stated this information was immediately passed to the FBI āfor further investigation.ā In reality, Alec Station not only failed to inform the Bureau of Mihdharās US visa, but also expressly forbade two FBI agents assigned to the unit from doing so.
ā[I said] āweāve got to tell the Bureau about this. These guys clearly are badā¦weāve got to tell the FBI.ā And then [the CIA] said to me, āno, itās not the FBIās case, not the FBIās jurisdictionā,ā Mark Rossini, one of the FBI agents in question, has alleged. āIf we had picked up the phone and called the Bureau, I wouldāve been violating the law. Iā¦wouldāve been removed from the building that day. I wouldāve had my clearances suspended, and I would be gone.ā
On January 15th, Hazmi and Mihdhar entered the US through Los Angeles International Airport, just weeks after the foiled Millennium plot. Omar al-Bayoumi, a Saudi government āghost employeeā immediately met them at an airport restaurant. After a brief conversation, Bayoumi helped them find an apartment near his own in San Diego, co-signed their lease, set them up bank accounts, and gifted $1,500 towards their rent. The three would have multiple contacts moving forward.