Congress asked the Justice Department to look into whether the seven-time Cy Young Award winner lied last February, when he testified under oath at a deposition and a public House hearing that he never took illegal performance enhancers.
That contradicted the sworn testimony of his former personal trainer, Brian McNamee, who said under oath that he injected Clemens with steroids and human growth hormone. Clemens last played in the major leagues in 2007, with the New York Yankees.
The Justice Department brought the case to a grand jury - which is based in Washington - after an 11-month FBI inquiry. A grand jury allows prosecutors to get sworn testimony from witnesses and collect documents. The investigation is being led by Assistant U.S. Attorney Daniel P. Butler, the prosecutor in the D.C. Madam case.
"It's standard operating procedure for a prosecutor, and it's probably been convened for a while," Clemens' lead lawyer, Rusty Hardin, said through spokesman Patrick Dorton.
The grand jury's involvement first was reported by ESPN.com.
Barry Bonds, baseball's career home run leader and a seven-time MVP, is scheduled for a March trial on charges he lied to a federal grand jury in 2003 when he denied knowingly using performance-enhancing drugs. That is part of a separate investigation in California that also ensnared star Olympic sprinter Marion Jones, who was sentenced to six months in prison for lying about her steroid use.
McNamee's lawyer, Richard Emery, said his client has not been called as a grand jury witness or received a subpoena. But Emery does expect McNamee to testify again.
"We will be cooperating. We've been in contact with the federal authorities for a year and a half," Emery said. "We look forward to the results, which we fully expect will show that Brian has been telling the truth all along."
Clemens' former teammate, Andy Pettitte, also has not received a subpoena. Pettitte corroborated some of McNamee's claims in a sworn statement to Congress.
Former New York Mets clubhouse attendant Kirk Radomski, sentenced to five years' probation last year after pleaded guilty to distributing steroids and laundering money, led investigators to McNamee.
McNamee told federal agents and baseball investigator George Mitchell that he injected Clemens more than a dozen times with steroids and HGH from 1998-01. Clemens' repeated denials of those accusations drew Congress' attention - and the former pitcher then made more denials under oath.
Clemens also filed a civil defamation suit against McNamee, a case pending in federal court in Houston.
Since the referral by Congress, federal investigators have been probing Clemens' past.
Shaun Kelley, owner of a Houston training center, said he had taken a polygraph test for FBI investigatorüThe investigation could pose an interesting ethics puzzle for President-elect Barack Obama's incoming team at the Justice Department.
Lawyer Lanny Breuer is expected to be announced any day as Obama's pick to lead the department's criminal division. Breuer was hired by Clemens last year as he prepared for the congressional investigation that has now resulted in a criminal investigation.
When he was hired in January 2008, Breuer said he was "honored" to join the legal team "representing one of the greatest pitchers and athletes in history."