Former Rep. Tom Davis, a Virginia Republican, was the chairman of the House Government Reform Committee on March 17, 2005, when McGwire repeatedly testified that he would not "talk about the past" when lawmakers asked him about performance-enhancing drugs.
In a telephone interview Monday with The Associated Press, Davis said he met for three hours with McGwire behind closed doors the day before that hearing. During that private session, according to Davis, McGwire made clear he had used steroids and wanted to say so but was worried he would face legal trouble by admitting it then.
"He would have been a nice head to put on somebody's mantel. So he basically took the Fifth (Amendment)," Davis said.
"It was very clear to everybody involved that he had taken steroids. Otherwise he would have gotten up there and denied it, but he couldn't. ... He looked ridiculous to most of the public, but he didn't have many good options. We put him in a pretty tight spot. He was candid and honest in our interrogation of him. He said: 'Some day, I'll tell the story."'
Davis added that he was turned down when he asked then-Attorney General Alberto Gonzalez to grant McGwire immunity in exchange for honest testimony to Congress.
McGwire Admits Using Steroids
What's the Legacy of Steroids in Baseball?
"I look back now, I wish I had got immunity," McGwire told the AP earlier Monday.
One of his lawyers, Mark Bierbower, said: "He did make an effort to fully disclose the steroid issue and to do so, we sought immunity, but it wasn't granted. It goes without saying that he walked out of the hearing knowing he had damaged his name, his reputation. But he did it to protect himself and his family, and we're very pleased for Mark he can say now what he wanted to say then."
McGwire - recently hired as hitting coach for one of his former teams, the St. Louis Cardinals - said Monday he used steroids and human growth hormone on and off for a decade, starting before the 1990 season and including the year he broke Roger Maris' single-season home run record by hitting 70 in 1998.
More from CBS Sports.com:
CBS Sports.com: McGwire's career statistics
CBS Sports.com: All-time career HR leaders
CBS Sports.com: All-time Single-season HR leaders
In 2005, Davis' congressional panel heard more than 10 hours of testimony from executives of baseball and its players' union; star players like McGwire, Sammy Sosa, Rafael Palmeiro and Jose Canseco; and others, including Donald Hooton, whose son committed suicide after using steroids.
Hooton was among the people McGwire called Monday to discuss his admission
"I do think he can make a good spokesman on this topic," Hooton said. "Who better to describe what it's like to go to the mountaintop and now to be at the depths of the deepest valley for a mistake you made several years ago?"
Photos: Steroids Greatest Hits
Video: McGwire Won't Name Names in 2005
Hooton, like Davis and many people around baseball, figured McGwire would make this sort of admission soon, given that he's now employed by the Cardinals.
"This is the right time. He knows he owes the baseball world an explanation," Davis said. "I think we all knew this. I don't think anybody's surprised by this. He was one of hundreds of players who used steroids during this time. ... This was so widespread. Had we not held these hearings and put the fear of God into baseball, it would still be going on."
McGwire himself noted Monday that the most recent image many people have of him is in a suit and tie, right hand raised to take an oath before Congress. Now he'll get the chance to be back in uniform as a coach - and as he returns to baseball, many wonder whether McGwire will one day be elected to the Hall of Fame.
In four appearances on the ballot, McGwire has hovered below 25 percent, not close to 75 percent needed for election.
Some baseball writers who vote for the Hall of Fame and have not backed McGwire in the past said Monday they didn't think Monday's news would change their stances on his candidacy - because they figured all along he had done steroids.
"That was a shocker, only in the fact he finally admitted it," said Bob Sherwin, a former Mariners beat writer and longtime member of the Seattle chapter of the Baseball Writers' Association of America. "I'm not sure this brings him any closer to the Hall's threshold, but it does end the speculation that did nothing but harm his eligibility."
Tom Verducci, with the MLB Network and a writer for Sports Illustrated said "this is not someone who dabbled in steroids - tried them once. This is someone who admitted most of his career was built on steroids."
There are those who are sure McGwire's coming clean will bring him more votes, although perhaps not enough to get into the Hall.
"I think McGwire's Hall of Fame chances will be enhanced by his long-overdue admission, but he was more than 275 votes short this time, said Rick Hummel of the St. Louis Post-Dispatch, a member of the writers' wing at Cooperstown. "That many voters are not going to change their minds."
Former player Willie McCovey, voted into the Hall of Fame in 1986, thinks McGwire belongs.
"Whether he took steroids or not, he did so much for baseball. He almost helped save baseball for a few years there," McCovey said. "I don't think he should be punished."