Brian McNamee, the former trainer of Roger Clemens and the key prosecution witness in the government’s case against the star pitcher, leaves federal court in Washington on Thursday after testifying in Clemens’s perjury trial. (Haraz N. Ghanbari/Associated Press)

The government’s star witness in the perjury trial of legendary pitcher Roger Clemens conceded Thursday that he confused critical dates, lied to federal agents and repeatedly changed elements of his story.

But Brian McNamee, a former strength coach for Clemens, stuck to the broad outlines of the narrative he has set out for years: that he injected Clemens with performance-enhancing drugs on several occasions during a three-year period starting in 1998.

Despite a second day of aggressive questioning by Clemens’s lead attorney, Rusty Hardin, McNamee generally appeared unflappable. Even as the trainer was forced to acknowledge that he mixed up dates and initially withheld physical evidence from law enforcement officials, he was often flip in his responses to Hardin.

“I’m just waiting for a question I can answer,” he said.

Hardin at times expressed disbelief about McNamee’s admitted exaggerations, noting the pressure the strength coach told Hardin’s investigators he was under to cooperate with law enforcement officials.

“I don’t know if I lied to them. I just embellished a little bit,” McNamee said.

“What in the world of Brian McNamee is the difference between an exaggeration and a lie?” Hardin asked.

“It’s taking a truth and exaggerating it,” McNamee answered.

Clemens is on trial for allegedly lying to Congress when he denied ever having taken performance-enhancing drugs in his 2008 testimony to a House panel. Lawmakers were responding to a report by former senator George Mitchell that named dozens of ballplayers, including Clemens.

McName is the only witness claiming first-hand knowledge of Clemens’s alleged use of steroids and human growth hormone.

In his cross-examination Thursday, Hardin tried to use McNamee’s words against him. The manuscript of an autobiography the strength coach dictated to an author describes McNamee injecting Clemens’s wife, Debbie, with HGH in preparation for a Sports Illustrated photo shoot. McNamee had testified Thursday that he never connected the photo shoot with Debbie Clemens’s interest in using HGH.

“Those are not your words?” Hardin asked, referring to the book, entitled “Death, Taxes and Mac” — a reference to McNamee’s nickname.

“Not exactly. I don’t speak like that,” McNamee said, explaining that the author actually penned the book.

Hardin also displayed a copy of the 2003 magazine photo to illustrate that the alleged injection of Clemens’s wife had to have occurred months earlier than McNamee previously testified.

Earlier in the week, McNamee told prosecutors that he became a reluctant source for federal investigators after they discovered large checks he had written to a steroid supplier for big league ballplayers. Law enforcement officials told McNamee he would not be a target of their investigation if he cooperated.

Throughout the day, Hardin suggested that McNamee was sticking to his story to ensure he goes “home free.”

“When he finds out he’s got a problem with his testimony, he’ll plug in some additional fact,” Hardin said.

As an example, Hardin pointed to McNamee’s account of why he saved medical waste from an injection he allegedly gave Clemens. McNamee testified that he kept the items because his wife was concerned that the strength coach would take the fall for the baseball star.

On Thursday, McNamee acknowledged that this explanation was a “moving target” and that he had previously tried to keep his estranged wife out of the story.