A former federal prosecutor's ambition led him to withhold evidence that could have helped the defense during the nation's first major terrorism trial after the Sept. 11 attacks, a government lawyer said at trial Wednesday.

Richard Convertino had designs on a seat in Congress and broke the law while prosecuting four North African immigrants accused of operating a "sleeper" terrorist cell, Daniel A. Schwager told jurors in opening arguments at Convertino's trial.

"Winning the ... case was everything to defendant Convertino," Schwager said. "This case is about truth and lies."

William Sullivan, Convertino's lawyer, said his client and a co-defendant in the case, former State Department investigator Harry Smith III, were only trying to bust up a terrorist cell and shouldn't be on trial.

"There was no reason to risk their careers or their lives on a tiny bit of evidence," Sullivan said.

Smith's lawyer, Thomas Cranmer, told the court that Smith had no motive to lie.

"He's a witness," Cranmer said. "No stake in the outcome of the case."

Convertino, 46, and Smith, 50, have pleaded not guilty to obstruction of justice, making false declarations before a court and conspiracy charges.

For two years, Convertino led the government's case against the four accused of operating a terrorist cell. Two of the four, Karim Koubriti and Abdel-Ilah Elmardoudi, were convicted in 2003 of conspiring to provide material support and resources to terrorists, and Convertino won praise from the Bush administration for his successful convictions.

However, a federal judge overturned the verdicts at the Justice Department's request after prosecutors discovered that some documents that could have aided the defense during the trial were not turned over by the government as required.

Convertino was the lead prosecutor in the case, and Smith helped in the investigation and testified for the government at the trial.

A lawyer for Smith was expected to make an opening statement later Wednesday. The trial in U.S. District Court in front of Judge Arthur Tarnow was expected to last about four weeks.