"The federal drug agent who spearheaded the BALCO steroids case accused Barry Bonds on Tuesday of thwarting his investigation by giving grand jury testimony that was "inconsistent with the facts."

Jeff Novitzky, the government's point man in a series of high-profile probes into steroids in sports, was the leadoff prosecution witness in the former Giants outfielder's trial on charges of lying under oath to a federal grand jury.

During his 2003 testimony before the grand jury probing the steroid ring operated by the Bay Area Laboratory Co-Operative in Burlingame, Bonds said he had never knowingly used BALCO drugs.

The testimony, which came after Bonds was granted immunity from prosecution for drug crimes, "absolutely" impeded the BALCO probe, Novitzky told a jury in San Francisco federal court.

Bonds' story directly contradicted the testimony of other athletes who had acknowledged to the grand jury that they had received undetectable designer steroids from BALCO and from Bonds' trainer Greg Anderson, Novitzky said. His account also conflicted with evidence seized in a federal raid on BALCO, the agent said.

"Mr. Bonds' testimony was inconsistent with the facts that had been gathered to that point," said Novitzky, a former Internal Revenue agent and now a Food and Drug Administration investigator on steroid use.

"We had to take a look at other testimony that was provided that was contrary to Mr. Bonds'," he said. "Which person is telling the truth here?"

Bonds has pleaded not guilty to five felony charges - four counts of lying under oath and one of obstruction of justice. Legal experts say that to win a conviction, the government must not only prove that Bonds lied, but that false statements were "material" - that they affected the course of the grand jury's investigation.

Novitzky, a tall man with a shaved head, testified that he was an IRS agent in 2002 when informants told him that BALCO President Victor Conte was dealing steroids to elite athletes, sometimes with the aid of Anderson, Bonds' friend and weight trainer.

During a "covert" phase of the investigation, Novitzky said, he began making late-night "trash runs" at BALCO, picking up the company's garbage and sorting through it for evidence."