CHESAPEAKE, Va. - In many ways, the trial of sniper suspect Lee Boyd Malvo comes down to his word against his word.

After his arrest last fall, Malvo told investigators in lengthy confessions that he pulled the trigger in all of the sniper shootings. But in meetings with a court-appointed psychologist this past summer, Malvo withdrew those confessions and said he was not the triggerman.

The question of which Malvo story is more credible could go to the jury by the end of this week. The teen-ager's defense team may rest its case today, bringing to an end two months of combined testimony in the guilt phases of the two sniper trials. The first trial, of John Allen Muhammad, ended with a conviction and death sentence.

But after all the testimony and more than a year of investigations since snipers terrorized the Washington region, the answers to two fundamental questions - What were the attacks about, and who pulled the trigger? - remain elusive. And it is becoming clear that those questions may never be answered to any satisfaction, because Muhammad remains steadfastly silent and Malvo has told wildly different versions of what happened.

"The truth is that we really don't know the truth," said Marvin D. Miller of Alexandria, former president of the Virginia Association of Criminal Defense Lawyers. "Down the road, something may come out that's more real, but I don't think it's there."

Prosecutors have proffered a multitude of motives, with three main reasons for the killings in play.

The first is that Muhammad wanted to terrorize his wife and reclaim the couple's three young children, who lived in Clinton, Md. The loss of those children had devastated Muhammad, witnesses said.

The second possible motive was to intimidate the government into giving the snipers a $10 million payoff to stop the killing.

And the third possible motive was for Malvo and Muhammad, heavily influenced by militant movies, video games and political rhetoric, to spark a revolution that would lead to an uprising by oppressed people against the government.

"John Muhammad pointed to the Sept. 11 terrorist attacks as evidence that the revolution is coming," defense psychologist Dewey G. Cornell testified this week. "He revealed to Lee that he felt they had been selected to carry out an important role - to free the oppressed black people of this country."

Malvo, 18, persisted in those beliefs for six months after his arrest Oct. 24 last year. He repeatedly claimed responsibility for all the shootings; referred to Muhammad, 42, as his father; and made anti-American drawings that spoke of the persecution of black people and the need for a violent overthrow of the government.

But a turning point finally came June 1, according to testimony. That was the first time, Cornell said, that Malvo acknowledged that he had been manipulated by Muhammad, and he began to give a different version of events. Malvo began to say that he had been only the spotter - not the shooter - in all but the final sniper shooting.

That, Cornell said, is the real Lee Malvo.

Prosecutors don't buy it. In cross-examining Cornell yesterday, lead prosecutor Robert F. Horan Jr. mocked Malvo's conversion, repeatedly referring to the "old Malvo" and the "new Malvo" as if they were two people. Horan asked which Malvo had committed various crimes, and which Malvo had been evaluated by Cornell.

But Cornell wouldn't play along. "You want to call it the old Malvo and the new Malvo, but I wouldn't say there's one day in which there's a new Malvo," said Cornell, a University of Virginia psychologist who spent 54 hours with Malvo this year. "I'd say there is a process he went through and is continuing to go through."

In weighing the credibility of Malvo's conflicting statements, jurors will probably consider his motive for making those remarks, legal experts say. The defense has claimed that his confessions, given in the weeks after his arrest, were the product of a mind still under the influence of Muhammad.

But experts also say that Horan will probably point out in his closing argument that Malvo has a very compelling reason to change his story: His life is on the line.

"It's so self-evident that this guy finally got the message that he had to find a way to get himself out of this," said Michael Greenberger, a University of Maryland law professor who was a high-ranking Justice Department official in the Clinton administration. He said Malvo's recantation would not likely affect the jury's decision in the guilt phase.

"My bet is the jurors, at least for the purposes of liability, are going to find that Malvo's confessions carry more weight than [what he said] post-psychiatric treatment, and they'll reach the conclusion that he pulled the trigger," Greenberger said.