Roger Clemens' former personal trainer cooperated with investigators looking into steroids in baseball under threat of prosecution, an assistant U.S. attorney said in a court filing today.

In a declaration filed in federal court in Houston as part of Clemens' defamation lawsuit against Brian McNamee, assistant U.S. attorney Matthew Parrella said he told McNamee he was not a target of the investigation but could become one if he failed to cooperate.

If McNamee made any false statements to federal investigators or to the group investigating doping in baseball, Parrella said, he could face prosecution.

McNamee told former Sen. George Mitchell's investigators that Clemens had used steroids and human growth hormone before they were banned by baseball.

Also filed Thursday were declarations from attorney Charles Scheeler, who worked with Mitchell, and Earl Ward, an attorney for McNamee. Both supported McNamee's claim that he faced prosecution if he did not help investigators.

Clemens has filed a defamation lawsuit against McNamee, who was quoted in the Mitchell report that he injected the seven-time Cy Young award winner more than a dozen times with steroids and HGH in 1998, 2000 and 2001. Clemens repeatedly has denied using performance-enhancing drugs.

McNamee's lawyers have argued that because he did not volunteer information about Clemens but was "coerced" into giving it under threat of prosecution, he is "immune from any defamation."

Lara Hollingsworth, one of Clemens' attorneys, said McNamee's conversations with Mitchell's people, a reporter and New York Yankees pitcher Andy Pettitte were defamatory and not covered by any immunity provisions.

"These affidavits ... don't contradict our prior positions," Hollingsworth said. "(McNamee) was not under court order, he was not under legal compulsion. He talked to Senator Mitchell because it was to his benefit to do so."