WASHINGTON — Brian McNamee injected Roger Clemens with performance-enhancing drugs more often than he previously claimed, the pitcher's former personal trainer said Wednesday at the start of their high-stakes testimony before Congress.

"I have helped taint our national pastime," McNamee said.

Clemens has denied the accusations that became public in December's Mitchell Report. The seven-time Cy Young Award winner repeated those denials under oath to the House Committee on Oversight and Government Reform, potentially setting of a Justice Department investigation that could lead to a criminal prosecution.

"I have never taken steroids or HGH," Clemens said. "No matter what we discuss here today, I am never going to have my name restored."

McNamee told baseball investigator George Mitchell that he injected Clemens 16 to 21 times with steroids and human growth hormone from 1998-01. McNamee also said that Yankees teammates Andy Pettitte and Chuck Knoblauch used HGH.

Committee chairman Henry Waxman questioned the credibility of McNamee, a former New York City police officer, saying he lied to police seven years ago during an investigation of a possible rape. And he challenged the credibility of Clemens.

"We have found conflicts and inconsistencies in Mr. Clemens' account. During his deposition, he made statements that we know are untrue," Waxman said, adding that some of Clemens' statements "are simply implausible."

Clemens and McNamee sat in the same room where Mark McGwire, Rafael Palmeiro and Sammy Sosa testified three years ago, along with a lawyer from Mitchell's firm between them.

Clemens stared straight ahead as McNamee spoke. Debbie Clemens, the pitcher's wife, sat behind her husband and listened as Waxman implicated her in HGH use, citing statements by Pettitte.

In his statement, McNamee made new accusations.

"I have had that opportunity to think about these events and consider the specific drug regimens we used," McNamee said. "As a result, I now believe that the number of times I injected Roger Clemens and Chuck Knoblauch was greater than I initially stated."

He also said: "Make no mistake: When I told Sen. Mitchell that I injected Roger Clemens with performance-enhancing drugs, I told the truth. I told the truth about steroids and human growth hormone. I injected those drugs into the body of Roger Clemens at his direction. Unfortunately Roger has denied this and led a full-court attack on my credibility. And let me be clear, despite Roger Clemens' statements to the contrary, I never injected Roger Clemens _ or anyone else _ with lidocaine or B-12."

McNamee also attacked the credibility of Clemens, with whom he worked closely for many years.

"While I liked and admired Roger Clemens, I don't think that I ever really trusted him," McNamee said.

By denying the accusations under oath, Clemens opened himself up to possible criminal charges if it is determined he made false statements or obstructed Congress.

"Coming into today's hearing, we have before us some very different stories. They're in many ways incompatible," said ranking Republican Tom Davis of Virginia, who presided over the committee's 2005 session with Mark McGwire and Rafael Palmeiro. "Someone is lying in spectacular fashion about the ultimate question."

Pettitte, excused from appearing Wednesday, issued a statement shortly before the hearing started and acknowledged using human growth hormone in 2004, in addition to his December admission that he took it for two days in 2002.

Last week, Pettitte was asked to discuss drug use in a deposition and affidavit before a congressional committee.

"In that affidavit, Andy informed the committee that in addition to the two shots a day of HGH he took for two days in 2002, he also took HGH for a one-day period in 2004, shortly preceding season-ending elbow surgery," his lawyer, Jay Reisinger, said in a statement released to The Associated Press.

"Andy had not previously mentioned this usage because he acquired the substance from his father, who had obtained it without Andy's knowledge in an effort to overcome his very serious health problems, which have included serious cardiac conditions."

In the affidavit, Pettitte said that Clemens told him nearly 10 years ago that he used HGH. Pettitte also said Clemens backtracked when the subject of HGH came up again in conversation in 2005, before the same House committee held the first hearing on steroids in baseball.

Pettitte said in the affidavit that he asked Clemens in 2005 what he would do if asked about performance-enhancing substances, given his admission years earlier. Pettitte said Clemens responded by saying Pettitte misunderstood the previous exchange in 1999 or 2000 and that, in fact, Clemens had been talking about HGH use by his wife in the original conversation.

On Tuesday, Clemens made the rounds on Capitol Hill one last time, wearing a gray pinstriped suit and squeezing face-to-face meetings into the busy schedules of committee members. He met with five lawmakers over a four-hour span Tuesday, on top of the 19 he saw Thursday and Friday.

"I enjoyed talking with him," said Rep. Diane Watson, D-Calif., who said the discussion included baseball stories and personal accounts about the Sept. 11 attacks. "It's always good to meet the person who is in the spotlight. ... I told him, 'This is not a trial.'"

But it might very well feel like one when Clemens and McNamee sit at the witness table and _ under oath _ offer what will surely be contradictory versions as to whether Clemens used steroids and HGH.

"I couldn't tell you who's telling the truth," Watson said.

Before Pettitte's affidavit came to light, Clemens got some help in his public relations push from a different ex-teammate Tuesday.

"I have never had a conversation with Clemens in which he expressed any interest in using steroids or human growth hormone," Jose Canseco said in a sworn affidavit, dated Jan. 22, that was submitted to the committee. "Clemens has never asked me to give him steroids or human growth hormone, and I have never seen Clemens use, possess or ask for steroids or human growth hormone."

In his affidavit, Canseco disputes various statements of McNamee's in the Mitchell Report. The affidavit also says "neither Senator Mitchell nor anyone working with him" contacted Canseco to attempt to corroborate things McNamee said.

Canseco's book about steroids in baseball, "Juiced," drew Congress' attention in 2005, leading to that year's hearing.

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AP Sports Writer Joseph White contributed to this report.