Assistant U.S. Attorney Daniel P. Butler left the grand-jury area shortly thereafter, about two hours after arriving at the court house. Butler would not comment on his way in or out.
Radomski led investigators to Clemens' former personal trainer, Brian McNamee, who told federal agents and baseball investigator George Mitchell that he injected Clemens more than a dozen times with steroids and HGH from 1998-01.
Radomski and McNamee figure to be among the primary witnesses against the seven-time Cy Young Award winner. McNamee, whose Tuesday meeting with federal prosecutors was postponed, last year provided government agents with syringes his lawyers said would link Clemens to drug use.
Congress asked the Justice Department to look into whether Clemens lied last February, when he testified under oath at a deposition and a public House hearing that he never took illegal performance enhancers.
The Justice Department brought the case to a grand jury after an 11-month FBI inquiry. A grand jury, whose proceedings are supposed to remain secret, allows prosecutors to get sworn testimony from witnesses and collect documents.