Agent Harry Samit testified under cross-examination at Moussaoui's trial that FBI headquarters' refusal to follow up "prevented a serious opportunity to stop the 9/11 attacks" that killed nearly 3,000 people.
Moussaoui is the only person charged in this country in the attacks.
The FBI's actions between Moussaoui's arrest on immigration violations on Aug. 16, 2001, and Sept. 11, 2001, are crucial to his trial because prosecutors allege that Moussaoui's lies prevented the FBI from thwarting or at least minimizing the Sept. 11 attacks. Prosecutors must prove that Moussaoui's actions caused the death of at least one person on 9/11 to obtain a death penalty.
But CBS News correspondent Jim Stewart reports that analysts, as well as Samit's testimony, suggest the FBI had enough trouble connecting dots even if Moussaoui admitted being an al Qaeda terrorist.
Under cross-examination by defense attorney Edward MacMahon, Samit acknowledged that he warned higher-ups and others in the government at least 70 times that Moussaoui was a terrorist, CBS News correspondent Barry Bagnato reports.
He said he sought Justice Department permission to seek a search warrant — and never got it.
"You needed people in Washington to help you out?" MacMahon asked.
"Yes," Samit said.
"They didn't do that, did they?"
Samit said no.
He confirmed under questioning that he had attributed FBI inaction to "obstructionism, criminal negligence and careerism" in an earlier report.
One FBI supervisor in Washington told Samit that he was getting unnecessarily "spun up" about his concerns over Moussaoui.
Moussaoui pleaded guilty in April to conspiring with al Qaeda to hijack aircraft and commit other crimes. The sentencing trial will determine his punishment: death or life in prison.
Moussaoui denies he had anything to do with 9/11 and says he was training for a future attack.