"Coleen Rowley, FBI agent. In May 2002 she wrote a caustic, 13-page letter to FBI Director Robert Mueller accusing FBI headquarters of hampering the investigation into alleged 20th hijacker Zacarias Moussaoui. She says officials at FBI headquarters resisted seeking search warrants and admonished agents who sought help from the CIA."
As they say.. google it.
Also I recall an interview with a California FBI agent. The interview was on a Fresno station in a talk show called the Radio Detective.
The host was retired LEO and had a private agency. He had worked with the FBI agent. IIRC the agent stated that they were aware of the goings on before 9/11 but Washington told them to back off.