Alexandria, Va. ? Zacarias Moussaoui’s lawyers on Monday turned another prosecution witness’s testimony into evidence that could help save the al-Qaida conspirator from execution.

Minneapolis FBI Agent Harry Samit alleged that “criminally negligent” bureau supervisors in Washington repeatedly blocked him from probing Moussaoui after his Aug. 16, 2001, arrest.

The refusal by FBI headquarters to let agents search the possessions of a suspect Samit exhaustively labeled a “terrorist” in e-mails meant that clues to other operatives weren’t discovered until after 9-11, Samit said.

“I accused the people at FBI headquarters,” Samit said, referring to an internal memo he wrote after the attacks.

“Of criminal negligence?” asked defense lawyer Edward MacMahon.

“Yes, sir,” Samit replied. “They obstructed.”

Moussaoui’s lawyers are trying to prove that he shouldn’t be executed based on the government’s claim that it could have stopped the 9-11 attacks if he had admitted the al-Qaida suicide hijack plot was under way.

Samit said he fretted to superiors that Moussaoui would hijack a London-to-New York flight and “he’d have the fuel onboard to reach D.C.”

The Secret Service was even alerted that Moussaoui had talked about being interested in seeing the White House and Empire State Building, he said.

“You moved heaven and earth to get a warrant to search this man’s belongings,” MacMahon said. The agent agreed.

In the first week of the Moussaoui trial, another FBI agent grudgingly admitted that the bureau knew in the early 1990s that al-Qaida had trained operatives in karate and knives and how to hijack aircraft.

The FBI also knew a decade ago that Osama bin Laden sent his personal pilot to the same Oklahoma flight school as Moussaoui, the agent said.