Asked pointedly by a committee member if McNamee was a credible witness, Mitchell said, "Mr. McNamee had an overwhelming incentive to tell the truth."
Mitchell reiterated that point when asked again only a few minutes later.
"We believe that the statements provided to us were truthful," he said.
Clemens released another statement of denial on Tuesday through Rusty Hardin, his Houston-based attorney.
"We have had no criticism of the Mitchell Report, only what it contains concerning Roger Clemens," Hardin said. "Senator Mitchell's testimony today shed no new light on this issue. Roger continues to adamantly deny that he ever used steroids or human growth hormone. He will do so again under oath before the House Committee, giving the public the opportunity to judge his credibility."
Reading from a statement that virtually repeated what he said a month ago, the former Senate Majority Leader told members of the House Oversight and Government Reform Committee that baseball's response to the problem of performance-enhancing drugs was "slow to develop and initially ineffective."
Considering that Mitchell's investigation was accomplished without subpoena power and the ability to compel testimony or produce documents, several members of the Committee doubted whether a further federal investigation into the matter would be fruitful.
"It is up to Congress to make that decision," Mitchell said, adding that he thought that MLB and its union were capable of dealing with the issue.
He recommended, as he had in his report, that it was time to move on.
"Everyone involved should be trying to bring this troublesome chapter to a close," Mitchell said. "Sometimes you have to turn the page and look toward the future."
Mitchell's testimony and his followup question-and-answer period took up a good portion of the session's opening hours.
The hearing also drew the attendance of Commissioner Bud Selig; Donald Fehr, the executive director of the Players Association; and several other high-level baseball officials -- Orioles owner Peter Angelos, Yankees president Randy Levine, Nationals president Stan Kasten and Bob DuPuy, Major League Baseball's president and chief operating officer.
Mitchell said he was confident that all of the recommendations he made in the Report to strengthen the current drug policy would ultimately be implemented. Last week, MLB established a Department of Investigations to internally review drug and other allegations made against Major League players and personnel, and made adjustments to tighten and change a number of clubhouse policies.
As far as McNamee was concerned, Mitchell said that McNamee had incentive to tell the truth because of an agreement with the federal government.
Under McNamee's federal agreement, Mitchell wrote and repeated for the Committee on Tuesday, "No truthful statements can be used against McNamee in any federal prosecution by that Office; if, however, he should be untruthful in any statements made pursuant to that agreement, he may be charged with criminal violations, including making false statements, which is a felony."
Thus, Mitchell added, McNamee had a lot to lose by lying to federal officials, who sat in on his three interviews with Mitchell and was advised "that he could face criminal charges if he made any false statements during these interviews, which were deemed by the prosecutors to be subject to his written agreement with the U.S. Attorney's Office."