This artist's rendering shows Zacarias Moussaoui, left, telling his defense attorneys Edward MacMahon, right, and Kenneth Triccoli, second from right, that he wants to testify during his sentencing trial, Thursday, March 23, 2006, at U.S. District Court in Alexandria, Va. After three rocky weeks, prosecutors rested their case for executing the allegeded al-Qaida conspirator but the witness who could prove most valuable for them has yet to take the stand: the defendant himself. [AP]

He said he was supposed to head a five-man crew that also would have included Richard Reid, a British citizen who tried to set off explosives in his shoes aboard a transatlantic flight two months after the Sept. 11 attacks.

Moussaoui was arrested a few weeks before the attacks for immigration violations, and eventually charged as an al-Qaeda co-conspirator. He pleaded guilty last April, and is on trial to determine whether he should be executed for his role.

No one else has been convicted in this country of involvement in the attacks, in which four hijacked planes were flown into the twin towers of the World Trade Center in New York, the Pentagon and a Pennsylvania field.

Moussaoui's statements contradicted what he told the court when he pleaded guilty. In his plea statement, he insisted that he had no involvement in Sept. 11 and that his attack on the White House was to come as part of a second wave of attacks. His testimony today was delivered in a calm and polite manner that was directly opposite the often angry and rambling discourses he has given in past court appearances.

Some U.S. government officials have called Moussaoui the "20th hijacker,'' referring to the fact that one of the hijacked planes had only four crew members instead of five. The name has stuck, and Moussaoui has been known to many as the 20th hijacker.

But in U.S. District Court in Alexandria this morning, Moussaoui, 37, denied being the 20th hijacker. Asked by his lawyer why he had signed his guilty plea as "the 20th hijacker," Moussaoui replied: "Because everybody used to refer to me as the 20th hijacker, and it was a bit of fun."

Questioned by defense lawyer Gerald Zerkin for less than 30 minutes, Moussaoui was asked: "Were you scheduled to be a pilot in the operation that was to be run on Sept. 11, 2001?''

He replied: "Yes. I was supposed to pilot a plane to hit the White House. I only knew about the two planes of the World Trade Center in addition to my own plane.''

Prosecutors have argued that Moussaoui should be executed because he lied to the FBI when he was arrested in August 2001 -- and Moussaoui appears to have made much of their case in his testimony today by admitting that he lied "because I wanted my mission to go ahead.''

When he pleaded guilty, Moussaoui signed a document admitting he had lied to allow his "al-Qaeda brothers to go forward" with the plot. Asked today who he meant, he named Mohamed Atta, the leader of the hijacking teams who piloted the first plane that hit the World Trade Center.

Looking at photographs flashed on a screen by prosecutors when they cross-examined him, Moussaoui said he knew most of the 19 hijackers, many of whom he had seen when he attended an al-Qaeda training camp and managed an al-Qaeda guesthouse in Afghanistan.

He said he knew other hijackers were in the United States in the summer of 2001 but did not have any contact with them. He said he was unsure if the other members of the crew for his plane were in the United States.

When he accepted a request from bin Laden and another top al-Qaeda leader to pilot a plane in a suicide mission, Moussaoui said he initially did not know the full extent of the operation. But he figured it would involve more than his own plane. "Being al-Qaeda, we don't do one operation. We do multiple strikes, so I knew there would be other operations,'' he said.

Earlier, Zerkin had told U.S. District Judge Leonie M. Brinkema that his client should not be allowed to testify because he does not recognize the court's authority and, "as an al-Qaeda member, he believes it is okay to lie."

But prosecutors said they considered Moussaoui a competent witness, and urged Brinkema to let him speak.

The judge acknowledged that Moussaoui, a French citizen, had "expressed his disdain of the United States," but said he had promised to testify within the parameters of what the court would allow. Moussaoui refused to swear on a Bible, but told Brinkema that he understood he was required to speak truthfully.

Brinkema also told the 12 jurors that they may begin deliberations in the case as early as Friday. Because the trial traditionally has recessed on Fridays, she said, she wanted to give jurors time to tell their employers that they may be needed in court every day this week.

"I cannot guarantee you, but I want to alert you that you might have this case for deliberation by Friday," she said.

Moussaoui had trained at a U.S. flight school -- as did the other hijackers -- before his arrest. He initially denied any wrongdoing.

The sentencing hearing is divided into two parts. Jurors first must vote on whether Moussaoui is eligible for the death penalty. If they find he is, a second phase would include testimony from family members of Sept. 11 victims. Jurors then would determine whether Moussaoui should be executed.

The trial began Feb. 6 with jury selection. Opening statements were March 6. Brinkema halted the proceedings for a week earlier this month, after a federal aviation lawyer wrongly shared information with several government witnesses.