The doubt Pettitte acknowledged on cross-examination Wednesday sounded like a significant step back from his testimony the day before that "Roger had mentioned to me that he had taken HGH."
Clemens, a seven-time Cy Young Award winner, is accused of lying to Congress in 2008 when he said he never used steroids or HGH.
Prosecutors had hoped Pettitte, with no apparent motive to lie, would reinforce a case that otherwise relies heavily on Brian McNamee, a former strength coach for both Pettitte and Clemens who says he injected Clemens with steroids and human growth hormone.
So Pettitte's concession weakens the prosecution's effort to prove Clemens guilty beyond a reasonable doubt, University of Iowa law professor James Tomkovicz said in an email.
"The prosecution's case seems to rest pretty heavily on Andy Pettitte's account, and if he is in genuine doubt about what Roger Clemens said to him, there would seem to be a good chance that the jurors will also be in doubt," Tomkovicz said.
Sounding more like a defense witness, Pettitte allowed that he could have misunderstood the conversation, said to have taken place in 1999 or 2000.
Is it fair to say there is a "50-50" chance that Pettitte misunderstood? Clemens' lawyer asked.
"I'd say that's fair," Pettitte replied.
After Pettitte's testimony, the defense asked the judge to strike it. The defense will file a brief to support its position.
The trial resumed Thursday with the second day of testimony from federal agent Jeff Novitzky, who helped spearhead drugs-in-sports investigations -- including those of baseball's Barry Bonds and star cyclist Lance Armstrong -- first while working for the Internal Revenue Service and now at the Food and Drug Administration.
Novitzky, tall and lanky with a shaved head, went through the evidence he collected from McNamee. That included needles, syringes, cotton balls and gauze, some of which he said contained steroids and HGH. Jurors saw the crushed Miller Lite beer can that McNamee said he used to store the materials.
McNamee has said he used the items while injecting Clemens. Novitzky said many were sent to a private lab for DNA testing. A lawyer for Clemens has called the evidence a "mixed-up hodgepodge of garbage."
Novitzky testified in a deep, authoritative voice, often looking at the jury while answering questions from the prosecutor. Jurors took notes and appeared more engaged than at other times in the trial.
The Clemens team didn't have the best of starts Thursday, even before entering the courtroom. Clemens forgot his wallet and didn't have his ID, so a court employee was summoned to confirm for security who he was. And Clemens' lawyer Rusty Hardin spilled coffee on his suit jacket and asked the judge if he could continue in a shirt and tie while the jacket dried. U.S. District Judge Reggie Walton obliged.
Harry Sandick, a former federal prosecutor who now defends white-collar cases, said Pettitte's 50/50 comment, at least by itself, will not be enough for the government to meet the burden of proving guilt beyond a reasonable doubt.
Sandick wondered whether prosecutors could have done a better job of preparing the jury for Pettitte's admission of some uncertainty. Letting the defense be the first to bring it out "makes it look as if the government was hiding something" even though the government had been aware of that for several years, Sandick said.
Back in 2008, Pettitte mentioned a few times in his own deposition to Congress that he might have misunderstood Clemens. At one point, under prodding from a congressional investigator, Pettitte said, "I don't think I misunderstood him." But even then, he added, "Six years later, when he told me that I did misunderstand him, you know, since `05 to this day, you know, I kind of felt that I might have misunderstood him."
Despite appearances, it's not universally agreed that Pettitte's wavering is fatal to the government case.
"Today's perceived concession, which the defense will no doubt hype in closing, while somewhat beneficial, is frankly akin to the standard `Law and Order'-type witness statement that a desired defense interpretation is `possible,"' said Ty Cobb, another former federal prosecutor now in private practice. Cobb said Pettitte's testimony on balance had been important to the prosecution.
Clemens told Congress in 2008 that Pettitte "misremembers" or "misheard" their conversation, and that's one of the 15 alleged false and misleading statements listed under the charge of obstruction of Congress. In his congressional deposition, Clemens said he was blindsided when he learned that Pettitte had used HGH on a couple of occasions.
"Shocked. Shocked. When I first -- when I first heard it, I said, `No way.' I was shocked," Clemens said. He used the word "shocked" 10 times within 11 sentences.
After his testimony Wednesday, Pettitte signed a baseball in the hallway and left the courthouse in a black SUV without commenting, free to continue his comeback attempt with the New York Yankees.
Prosecutors did get a minor victory later Wednesday, when the judge allowed them to address in slightly greater detail the overall problem of performance-enhancing drug use in baseball to support Congress' involvement in the matter. Clemens' lawyers contend the 2008 hearing was a "show trial" of Clemens and not meant to serve a legislative purpose.
The government brought back congressional staffer Phil Barnett, whose testimony was interrupted Tuesday by Pettitte's arrival as a witness from out of town. Barnett, who was majority staff director of the committee that held the 2008 hearing, mentioned former major league stars Rafael Palmeiro and Chuck Knoblauch for the first time as players who have been associated with performance-enhancing drug use.
