ALEXANDRIA, Va. – Al-Qaida conspirator Zacarias Moussaoui testified Monday that he and would-be shoe bomber Richard Reid were supposed to hijack a fifth airplane on Sept. 11, 2001, and fly it into the White House.

Moussaoui’s testimony on his own behalf stunned the courtroom. His account was in stark contrast to his previous statements in which he said the White House attack was to come later if the United States refused to release an Egyptian sheik.

On Dec. 22, 2001, Reid was subdued by passengers when he attempted to detonate a bomb in his shoe aboard American Airlines Flight 63 from Paris to Miami. The plane was diverted to Boston, where it landed safely.

Moussaoui told the court he knew the World Trade Center attack was coming and he lied to investigators when arrested in August 2001 because he wanted it to happen.

“You lied because you wanted to conceal that you were a member of al-Qaida?” prosecutor Rob Spencer asked.

“That’s correct,” Moussaoui said.

Spencer: “You lied so the plan could go forward?”

Moussaoui: “That’s correct.”

The exchange could be key to the government’s case that the attacks might have been averted if Moussaoui had been more cooperative following his arrest.

Moussaoui told the court he knew the attacks were to take place some time after August 2001 and bought a radio so he could hear them unfold.

Specifically, he said he knew the World Trade Center was going to be attacked, but he asserted he was not involved in that part of the plot and didn’t know the details.

Nineteen men pulled off the Sept. 11 attacks on New York and Washington in the worst act of terrorism ever on U.S. soil.

“I had knowledge that the Twin Towers would be hit,” Moussaoui said. “I didn’t know the details of this.”

Sally Regenhard, whose firefighter son Christian died at the World Trade Center, said “at least there would have been a chance” to head off the attacks if Moussaoui had told investigators in August 2001 what she heard him admit in court Monday.

“I was convinced that this man was only a heartbeat away from taking the controls of a plane,” she said.

Asked by his lawyer why he signed his guilty plea in April as “the 20th hijacker,” Moussaoui replied: “Because everybody used to refer to me as the 20th hijacker and it was a bit of fun.”

Before Moussaoui took the stand, his lawyers made a last attempt to stop him from testifying.

Defense attorney Gerald Zerkin argued that his client would not be a competent witness because he has contempt for the court, only recognizes Islamic law and therefore “the affirmation he undertakes would be meaningless.”