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FBI agents asked Barry Bonds' ex-girlfriend not to cooperate in Major League Baseball's steroid probe while a federal grand jury investigates whether the Giants slugger lied under oath about drug use.

Attorney Martin Garbus said Friday that agents asked Kimberly Bell not to assist former Senate Majority Leader George Mitchell in the independent investigation he is heading.

"I would say they want to protect their own prosecution," Garbus said. "The consequence is, yes, they are impeding the Mitchell investigation."

The FBI declined to comment Friday and Mitchell did not immediately return calls for comment.

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Garbus said Mitchell wrote him May 31 and demanded "that Ms. Bell cooperate with my independent investigation of alleged steroid and performance-enhancing drug use in major league baseball."

Bonds would be entitled to learn whatever the former girlfriend tells Mitchell, Garbus said. If that information conflicts with what she told federal authorities, Bonds could use it to undermine her credibility in court.

"She might say something that the feds would rather her not," Garbus said.

Garbus said he was mulling whether to keep his client mum or comply with Mitchell's demand.

Bonds' lawyer, Michael Rains, also speculated agents want to keep Bell from talking to Mitchell because she could hinder their case with inconsistent answers.

"Maybe they realize when Kim Bell starts answering questions, it's gonna become clear that she first tried to extort Barry for money, that she changed her stories about various things and has changed it since then and will change it again," Rains said.

Rains said Bell allegedly "heard Barry talk about using things, noticed he had acne and was subject to mood swings." Acne and mood swings can be side effects of steroid use.

Rains said it was unlikely Bonds, who is second on the career home run list, would cooperate with Mitchell while a federal grand jury perjury probe continues, because federal agents could use those responses against him.

"I'm not frankly wanting to cooperate with the federal government these days," Rains said. "If his statement can be insulated from federal grand jury proceedings, I'd consider it. I don't think there is any way to do that."

Meanwhile, federal authorities are turning up the heat against athletes suspected of using or dealing in performance-enhancing drugs.

Agents searched the Scottsdale, Arizona, house of Diamondbacks reliever Jason Grimsley on Tuesday for human growth hormone, steroids and amphetamines. Arizona quickly released him from the team.

Grimsley Reportedly Asked to Wear Wire in Bonds Probe

Rep. Cliff Stearns of Florida, chairman of the Commerce, Trade & Consumer Protection Subcommittee, said Friday that he was considering calling a hearing on the use of human growth hormone in sports.

The drug prescribed to spur growth in small children and help adults with pituitary problems is used illegally by athletes to boost energy and strength and cannot be detected by baseball's urine tests.

"Once again we learn of the use of performance-enhancing drugs in major league baseball and the failure of MLB to rid the sport of drugs," Stearns said.

The Grimsley search underscored a shift to target athletes in the probe instead of suppliers and chemists.

Yet the nationwide probe that focused on a San Francisco-area drug lab is not over, said U.S. Attorney Kevin Ryan. Five Bay Area Laboratory Co-Operative defendants have pleaded guilty to distributing or developing steroids, some of which were undetectable in drug tests.

Bonds is being investigated for allegedly lying about whether he used the performance-enhancing drug known as "the clear" during 2003 grand jury proceedings that led to the indictment of BALCO's president, Bonds' personal trainer and others. Bonds said he thought he was using flaxseed oil.

After the BALCO investigation, baseball toughened its performance-enhancing drug testing, which now includes tests for amphetamines but not human growth hormone, which only can be detected through blood tests.

In the aftermath, baseball commissioner Bud Selig brought in Mitchell to look into the use of performance-enhancing drugs in baseball.

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