WASHINGTON – During another seven grueling hours of cross-examination that frustrated all sides, Roger Clemens’ accuser explained the evidence he kept in a beer can — and why his story about it has changed.

Brian McNamee was on the stand Thursday for a fourth day in the perjury trial of the seven-time Cy Young Award-winning pitcher, holding firm to his testimony that he injected Clemens with steroids from 1998 to 2001 and human growth hormone in 2000.

But Clemens’ longtime strength coach again conceded that his memory of some details has evolved over the years, and that he initially told some lies during the drugs-in-baseball investigation conducted by federal agents and former Sen. George Mitchell.

Whether the jurors were still keeping track is another matter: They again expressed concern about the agonizingly slow pace of a trial that still has weeks to go, and the judge opined that Clemens’ lawyer was “confusing everybody.”

“At this pace,” U.S. District Judge Reggie Walton said, “I’ll guess we’ll be here forever.”

Clemens’ lawyer, Rusty Hardin, tried to exploit McNamee’s inconsistencies, even if it meant taking the case far afield from the principal issue of whether Clemens actually used performance-enhancing drugs. The former baseball star is accused of lying when he testified to Congress in 2008 that he never used steroids or HGH.

The day’s testimony ended at a tantalizing moment. After some 19 hours on the stand, McNamee was being challenged by Hardin over the needle and other waste kept in a Miller Lite can after a steroids injection McNamee said he gave Clemens in 2001. The government is expected to show the waste contains Clemens’ DNA.

McNamee indicated to Congress in 2008 that he kept the evidence primarily because he was starting to distrust Clemens, but he told the jury earlier this week he kept it because his wife had starting nagging him to do something to protect himself from being a fall guy in case he ever got caught.

McNamee said Thursday he had hoped to keep his wife out of the story. His change of heart came as he and his wife are going through a contentious divorce.

“Now she’s involved,” McNamee said. “She’s got to take responsibility for her action.”

McNamee said the beer can came from the recycling bin in Clemens’ apartment, while conceding that he’d never seen Clemens drink a light beer. Hardin insinuated that McNamee manufactured the evidence after Clemens’ televised denials of steroids use.

Hardin’s aim is to portray McNamee as a serial liar, and he appeared to have some success this day.

“Did you ever tell Sen. Mitchell that you injected Roger Clemens approximately four times in the rear over a two-week period in 1998?” Hardin asked.

“That’s possible,” McNamee answered.

“If you did tell him would that be a lie?” Hardin asked.

“Yes, it would,” said McNamee, who testified this week that he injected Clemens about eight to 10 times during Clemens’ 1998 season with the Blue Jays.