WASHINGTON – He has spent more than four years in an isolated cellblock he calls his cave under guard so tight his meals get slipped through a slot in the door.

Now Zacarias Moussaoui is finally going on trial. The confessed al-Qaida conspirator pleaded guilty last April to six conspiracy counts related to the Sept. 11 attacks.

Today, the penalty phase of his case begins to determine whether Moussaoui, a defiant French Moroccan, will die by lethal injection or spend the remainder of his life in prison.

That Moussaoui is an adherent of al-Qaida, despises the United States and would be thrilled to fly an airplane into a building filled with American civilians is beyond dispute. He has proclaimed those views loudly in several court appearances. But he has also said that he was not part of the Sept. 11 plot and does not deserve or want to be executed.

But the case, so far the only U.S. public court prosecution stemming from the attacks, is significant for another reason: It shows how difficult prosecuting suspected terrorists can be in a war against Islamic extremists that’s shrouded in secrecy.

Moussaoui’s case has been slowed by the sheer volume of evidence in the Sept. 11 investigation, fights over the death penalty, questions about his mental competency and by his repeated attempts to fire his attorneys.

But the longest delays have been caused by legal impasses over Moussaoui’s right to use classified information in his defense.

"You’re dealing increasingly with information that the government does not want to see the light of day," said Saul Pilchen, a Washington attorney who has written on the use of classified information in criminal trials.

The press for secrecy is creating a dilemma that shadows Moussaoui’s case: Should terrorism suspects be treated as criminals, prisoners of war, or potential intelligence sources?

Moussaoui’s trial to determine his punishment, to be held in a federal courthouse a few miles from where a hijacked jetliner crashed into the Pentagon on Sept. 11, 2001, will give the government a long-sought forum to showcase the FBI’s investigation into the attacks.

Under federal law, violations that carry the death penalty require separate proceedings to determine first whether someone is guilty and then what the penalty should be. With his guilty plea, the issues and arguments that would have been at the heart of any trial over his guilt will now form the basis of the sentencing phase of the proceeding, which will resemble in almost every way an ordinary trial over guilt.

Moussaoui and his court-appointed lawyers, with whom he apparently does not communicate, have offered different principal defenses as to why he should not be executed. In motions before the court, the defense lawyers have indicated they will seek to present testimony from medical professionals that Moussaoui is mentally unstable.

Moussaoui has had several seemingly irrational outbursts in previous hearings that caused Judge Leonie Brinkema to revoke her permission for him to defend himself. She has, however, ruled him competent to stand trial and to enter his plea.

Moussaoui, 37, has asserted that he had nothing to do with the Sept. 11 attacks. He was in jail at the time, having been arrested weeks before on suspicion of immigration violations when he was a student pilot in Minnesota.

FBI and immigration agents arrested him after telephone tips from two suspicious program managers at the Pan Am International Flight Academy in Eagan, Minn. He had paid more than $8,000 to take lessons in a 747 flight simulator.

When he asserted in court that he was eager to be a suicide bomber, he said he was meant to be a part of a second wave of attacks by airplanes on public buildings.

Defense lawyers seeking to raise doubts that Moussaoui’s help would have stopped the attacks could make public embarrassing new revelations about U.S. intelligence lapses.

Prosecutors will argue they can prove the government could have saved at least one life if Moussaoui had told federal agents what he knew about al-Qaida’s plans.

They likely will present details of how top al-Qaida officials planned the attacks and how Moussaoui’s activities mirrored the hijackers’.

Defense lawyers are expected to cite interrogations of key al-Qaida captives overseas who reportedly have said that Moussaoui was left out of the planning for the attacks, and also to show that the government knew more about the attack plot than he did.

Dozens of lawyers hired by the defense have combed through thousands of CD-ROMS filled with bank records and reports on 166,000 non-classified FBI interviews. The defense also has received data from more than 100 computer hard drives seized from suspected al-Qaida operatives around the world.

Some legal experts say the Moussaoui case will smooth the way for more public terrorism prosecutions in the future.

The Justice Department says it has prosecuted in military court more than 400 people, mostly U.S. residents, on what it considers terrorism-related charges.

But Moussaoui is among only a few of thousands of foreign detainees to face prosecution in a public court. Most others were captured overseas, designated as "enemy combatants," and held in Afghanistan, at the U.S. naval base at Guant