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A forensic expert on Tuesday cast doubt on evidence being used against former baseball pitching ace Roger Clemens in his perjury trial, saying medical waste the prosecution says shows his use of performance enhancing drugs could have been contaminated.

Clemens, 49, is on trial for a second time on federal charges of lying in 2008 to a U.S. congressional committee when he denied use of performance-enhancing drugs.

The House of Representatives Committee on Oversight and Government Reform was investigating drug use in Major League Baseball.

The core of the government's case against the seven-time winner of the Cy Young Award for best pitcher has been a batch of medical waste that Clemens' ex-trainer Brian McNamee says came from a 2001 injection of Clemens with anabolic steroids.

"I think the government's conclusions are overreaching with regards to the interpretation of this evidence," said defense witness Bruce Goldberger, a forensic toxicologist.

Goldberger said the evidence was "lacking" because cotton balls and a needle may have been contaminated from other items in the batch of medical waste. McNamee has admitted that he kept medical waste from injections of other baseball players in the same place.

Two forensic analysts previously testified that they had found steroids on the items.

McNamee has testified that he personally administered shots of steroids and human growth hormone to Clemens between 1998 and 2001, and stashed needles, gauze, and a broken steroid ampoule and other waste in a Miller Lite beer can and a Fedex box.

BOLSTERING CLEMENS' DEFENSE

In other testimony on Tuesday, former pitcher Mike Boddicker said he saw Clemens being injected with vitamin B-12 in 1989 or 1990 by a trainer when the two played for the Boston Red Sox.

Clemens has said McNamee injected him with shots of a liquid form of vitamin B-12, used to help ward off sickness, and the anesthetic lidocaine, not with anabolic steroids.

Boddicker said trainers on the team regularly administered shots of the vitamin, countering testimony by prosecution witnesses that vitamin B-12 was kept under lock and that McNamee would not have had authority to administer it.

Joe Angel, a broadcaster for the Baltimore Orioles, also shored up Clemens defense, testifying he saw Clemens at a golf course on the day of a 1998 pool party at the Florida home of Jose Canseco, a Toronto Blue Jays teammate of Clemens and an admitted steroid user.

Prosecutors charge that Clemens lied to Congress when he testified in 2008 that he did not attend the party.

CLEMENS' SUCCESS BY HARD WORK

Clemens' lawyers have tried to depict him as a hard-working pitcher whose stunning late-career success was the product of dedication and smart pitching, not performance-enhancing drugs.

Clemens won his seventh Cy Young Award in 2004 - the summer he turned 42 - in his first season with the Houston Astros, after ending the season with an impressive 2.98 earned run average.

Rohan Baichu, a former Yankees massage therapist and another defense witness, testified that he did not notice changes in Clemens' body in the several years he gave him deep tissue massages.

Defense attorney Rusty Hardin has argued that if Clemens had used steroids he would have shown some side effects such as acne, weight gain or enlarged breasts.

Hardin said the defense team could rest their case on Friday, allowing jury deliberations to begin next week. The trial is now in its eighth week.

Clemens' first trial ended last year in a mistrial.

(Reporting By Lily Kuo; Editing by Cynthia Osterman and Vicki Allen)