ALEXANDRIA, Va.- Prosecutors rested their death-penalty case against Al Qaeda terrorist Zacarias Moussaoui yesterday after a former FBI agent testified that investigators might have been able to hunt down September 11, 2001, hijackers if the defendant had confessed before the attacks.

When Moussaoui pleaded guilty last year, he gave details that could have helped identify 11 of the 19 hijackers if he had been forthcoming when he was arrested in the month before the attacks, former agent Aaron Zebley said.

After Mr. Zebley’s testimony, U.S. District Judge Leonie Brinkema held a closed hearing and sent the jury on a half-hour break. Leaving the courtroom, Moussaoui made clear he intended to take the stand in his own defense.

“I will testify, Zerkin, whether you want it or not,” he said, referring to one of his lawyers, Gerald Zerkin. Moussaoui has consistently refused to cooperate with his lawyers.

Mr. Zebley testified that Moussaoui’s admission that he received more than $14,000 in wire transfers from a man using the name Ahad Sabet could have allowed the FBI to go through Western Union, cell phone, calling-card, and motor vehicle records, as well as leases, to identify most of the hijackers.

Judge Brinkema’s rulings prevented Mr. Zebley from explicitly speculating what the FBI actually would have accomplished had it known this information when Moussaoui was arrested in August 2001. But the clear implication was that Moussaoui’s refusal to give a timely confession thwarted some of the FBI’s best opportunities to prevent or at least minimize the September 11 attacks.

“We could have set about finding the hijackers,” Mr. Zebley said.

As it was, certain details did not come out until Moussaoui pleaded guilty in federal court in April.

On cross-examination, defense lawyer Edward MacMahon asked why the urgent pleadings of the FBI agent who arrested Moussaoui for an all-out investigation were ignored.

“The FBI has to have a confession … before anybody listens?” Mr. MacMahon asked.

Mr. Zebley replied that the arresting agent, Harry Samit, did not know all the details that were later included in Moussaoui’s confession.

The defense also argues that it’s not relevant to look at what the results might have been if Moussaoui had confessed when arrested, because he had a right to remain silent in the face of interrogation.

Earlier in the trial, Mr. Samit said he spent four weeks warning his bosses about the radical Islamic student pilot. He said bureaucratic resistance to a thorough investigation blocked “a serious opportunity to stop the 9/11 attacks.”

Moussaoui is the only person in this country charged in the attacks that killed nearly 3,000 people when Al Qaeda flew hijacked planes into the World Trade Center, the Pentagon and a Pennsylvania field. He pleaded guilty in April to conspiring with Al Qaeda to hijack aircraft and commit other crimes, but he denies a specific role in 9/11. The sentencing trial now under way will determine whether he is executed or imprisoned for life.