The National Security Agency (NSA) in December 1999 had listened to an important call made from an Al Qaeda safe house in Yemen, which they began monitoring since 1996. This call was made from an Al Qaeda operative, nicknamed Khallad (real name Walid Bin Attash) to a person known as Khalid al-Mihdhar. In this call, Khallad told al-Mihdhar that a very important summit meeting was to be held in Kuala Lumpur, Malaysia on January 3 2000. He told al-Mihdhar to bring Nawaf al-Hazmi and his brother Salem al-Hazmi along. The NSA contacted the CIA about this meeting, to which the CIA contacted Malaysian authorities to take photographs of this meeting. Pictures of Khalid al-Mihdhar, Nawaf al-Hazmi and Walid Bin Attash were taken and shared with CIA officials at the Bin Laden Issue Station (Alec Station) and the counter-terrorism center.

However, none of this information, which also included al-Mihdhar's and al-Hazmi's dual US visas were shared with anyone outside of the agency. The FBI found out and tried to warn headquarters, but were told not to share this information with anyone without CIA authorization. There was a coordinated effort from the CIA to intentionally with hold information from the FBI and the State Department about Khalid al-Mihdhar and Nawaf al-Hazmi, the question is, why?

OIG Report On CIA Accountability With Respect To The 9/11 Attacks:

https://irp.fas.org/cia/product/oig-911.pdf