The FBI may have tracked 11 of the 9/11 hijackers if the only person to be charged for the attacks had co-operated when first arrested, a court has heard.
Aaron Zebley (centre) testifies as Moussaoui (front) listens
Ex-FBI agent and now prosecutor Aaron Zebley said a major probe would have been launched if Zacarias Moussaoui had provided information in August 2001.
Mr Zebley was testifying as the prosecution rested its case in its argument for the death penalty.
Moussaoui pleaded guilty last April to six charges of conspiracy.
As he left the court in Alexandria, Virginia, at the end of the prosecution's argument, Moussaoui, who has consistently refused to co-operate with his court-appointed defence lawyers, shouted at one of them: "I will testify... whether you want it or not."
'Right to silence'
Mr Zebley had testified that the FBI could have tracked down 11 of the hijackers via phone cards, wire transfers and by investigating flight schools if Moussaoui had said a plot was under way.
"You've got 11 different names. We could have set about finding them, of course, shared information with the intelligence community and... federal law enforcement," Mr Zebley said.
He said Moussaoui did later provide information on the 11 after pleading guilty.
Defence lawyer Edward MacMahon argued that no such investigation would have been launched, saying the FBI had failed to track down two hijackers it did have knowledge of.
The defence also argues that Moussaoui, a 37-year-old Frenchman of Moroccan origin, did not have to supply information as he had the right to remain silent.
Moussaoui admits conspiracy to hijack aircraft and commit other crimes but denies a direct role in the 11 September attacks that killed almost 3,000 people.
The jury only has two choices - the death penalty or life imprisonment.