An FBI agent has told the sentencing trial of Zacarias Moussaoui that his attempts to warn of a possible hijack plan were obstructed by his superiors.
FBI agent Harry Samit (r) said he had warned of a possible hijack plot
Harry Samit, who arrested Moussaoui a month before the 9/11 attacks, warned bosses on 18 August 2001 he thought Moussaoui was planning a terror act.
But, he told the court, FBI head office refused his pleas to obtain a search warrant for the detainee's belongings.
Prosecutors allege Moussaoui's lies on the 9/11 plot prevented the FBI acting.
But Moussaoui's defence argues that nothing he said after his arrest would have made any difference because the FBI was too bureaucratic to respond quickly to a threat.
A self-confessed member of al-Qaeda, Moussaoui has denied being part of the 9/11 plot but says he was part of a broader conspiracy to use airplanes to strike the White House.
He is the only person to face trial in the US in connection with the attacks on New York and Washington.
Giving evidence in Virginia, Mr Samit told the court his warnings over Moussaoui had been dismissed by his superiors at FBI headquarters in Washington.
He said he had alerted his bosses immediately after questioning Moussaoui, because he believed the suspect was conspiring to commit a terrorist act.
He said he had voiced concern over a possible hijack plot, based on the fact Moussaoui had been learning how to fly a 747 airliner, owned a portable GPS navigation system and was an Islamic fundamentalist.
But despite his entreaties, FBI headquarters refused to open a criminal investigation and obtain a search warrant for Moussaoui's possessions, Mr Samit said.
"You needed people in Washington to help you out?" asked defence lawyer Edward MacMahon. "They didn't do that, did they?"
Mr Samit replied: "No."
He confirmed under questioning that in a report after the 9/11 attacks he had blamed the FBI's inaction on "obstructionism, criminal negligence and careerism".
Moussaoui's trial resumed on Monday after a week-long delay while Judge Leonie Brinkema considered whether to allow prosecution testimony from aviation experts.
It followed revelations that a government lawyer had improperly coached four witnesses.
Moussaoui, a 37-year-old Frenchman of Moroccan origin, pleaded guilty last April to six charges of conspiracy.
The prosecution is seeking the death penalty on the grounds his silence about the 9/11 plot after his arrest prevented the FBI thwarting the attacks.
He was detained shortly before the 9/11 attacks after arousing suspicion at a flying school. He initially told federal agents he was training as a pilot only for personal enjoyment.
If Moussaoui is spared the death sentence, he will spend the rest of his life in prison. The sentencing trial is expected to last up to three months.