WASHINGTON - Speaking softly, nervously, and in detail, Brian McNamee testified about the life-changing moment when, he said, he first injected Roger Clemens with steroids.

The government's star witness in the Clemens perjury retrial took the stand Monday and told the jury that he injected one of baseball's most successful pitchers with steroids about eight to 10 times when they were with the Toronto Blue Jays in 1998.

"I knew what I was doing was illegal," McNamee said. "I wish to God I could take it back."

Clemens is charged with lying to Congress when he testified in 2008 that he had never used steroids or human growth hormone.

Monday's testimony got to the heart of the government's case: McNamee is the only person who will claim firsthand knowledge of Clemens' using performance-enhancing drugs.

In his thick New York accent, McNamee recalled how he met Clemens when McNamee was the strength and conditioning coach of the Blue Jays during the 1998 season, and how he was asked by Clemens to come to Clemens' apartment in the Blue Jays' Skydome stadium after a game one day in June.

McNamee said he found steroids, alcohol, needle, and gauze laid out in the bathroom. He said he felt "a little uncomfortable" while preparing the shot because he'd never done anything like it before. He said he then walked into Clemens' bedroom.

"Roger pulled down his pants, exposing his right buttocks cheek to me," McNamee said. A few seconds later, Clemens said he was ready. McNamee said he then "plunged the fluid into his buttocks."

After it was done, they "exchanged pleasantries," according to McNamee.

"That," McNamee concluded, "was the first time I injected Roger Clemens."

McNamee said he didn't feel good about the moment, but he got the sense that Clemens "wasn't good at doing the 'booty shot."'

"I did it," McNamee said, "because I wanted to help and I wanted to keep my player safe. ... I wasn't under the assumption that was the first time he did that."

McNamee also recited details of another injection later in the season, one he gave Clemens in a supply room in the Tampa Bay clubhouse.