WASHINGTON — The FBI has begun investigating whether Roger Clemens lied to Congress when he denied taking steroids, officials said today in the case of another baseball star snared in a long-running inquiry into drug use by professional athletes.

Clemens, the All-Star pitcher and seven-time Cy Young Award winner, maintains he has never used steroids or other performance-enhancing drugs. His former personal trainer, Brian McNamee, claims otherwise, testifying that he injected Clemens with human growth hormones and steroids at least 16 to 21 times between 1998 and 2001.

FBI agents in Washington opened the case a little more than two weeks after both Clemens and McNamee appeared at the same House hearing on February 13, each accusing the other of lying. Three law enforcement officials, speaking on condition of anonymity because of the sensitivity of the case, confirmed the inquiry had begun.

The inquiry, opened following a request yesterday from the House Oversight Committee, could result in perjury or obstruction of justice charges against Clemens. Lawmakers did not ask for a similar investigation of McNamee.

Clemens was first identified as taking steroids in a December report by a former Senate Democratic leader, George Mitchell, who looked at drug use in baseball. The Mitchell Report was the first public accounting of McNamee’s allegations that he injected Clemens with HGH and steroids. McNamee has since said he now thinks those numbers are too low.

Two of Clemens’s former New York Yankees teammates, Andy Pettitte and Chuck Knoblauch, have both acknowledged that McNamee was correct when he said they used performance enhancers.