Agent Harry Samit testified under cross-examination at Moussaoui's trial on Monday that FBI headquarters' refusal to follow up "prevented a serious opportunity to stop the 9/11 attacks" that killed nearly 3000 people.
The FBI's actions between Moussaoui's arrest on immigration violations on 16 August and 11 September, 2001, are crucial to his trial because prosecutors allege that Moussaoui's lies prevented the FBI from thwarting or minimising the 9/11 attacks.
Prosecutors must prove that Moussaoui's actions caused the death of at least one person on 9/11 to obtain a death penalty.
The defence argues that nothing Moussaoui said after his arrest would have made any difference to the FBI because its bureaucratic intransigence rendered it incapable of reacting swiftly to Moussaoui's arrest.
Under cross-examination by defence attorney Edward MacMahon, Samit acknowledged that he predicted in an 18 August, 2001, memo that Moussaoui was a radical Islamic terrorist in a criminal conspiracy to hijack aircraft. Moussaoui ended up pleading guilty to two specific counts that Samit had explicitly predicted in his 18 August memo.
Despite Samit's urgent pleadings, FBI headquarters refused to open a criminal investigation and refused Samit's entreaties to obtain a search warrant.
"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-Qaida 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.
Resumption of the trial followed a one-week delay while US District Judge Leonie Brinkema considered whether to allow testimony by aviation witnesses considered vital to the government's effort to secure the death penalty for Moussaoui.