U.S. Olympic gold medalist Marion Jones, one of the world's best known athletes, on Thursday joined a growing list of sports stars called to testify before a federal grand jury probing doping allegations against San Francisco area nutrition lab BALCO.

Jones, accompanied by a bodyguard and two other men, spent about three hours behind closed doors in the San Francisco federal courthouse, and left without discussing the case.

"I can't make any comment, you guys," she told a group of reporters staked out in the courthouse. "I wish I could but I can't."

Also testifying behind closed doors in the case that has riveted the sports world were three players for the Oakland Raiders football team, running backs Tyrone Wheatley and Chris Heatherington and defensive tackle Dana Stubblefield, who was named the National Football League's best defensive player in 1997 when he was with the San Francisco 49ers.

None of the athletes spoke to reporters, although Wheatley was involved in a minor altercation with a photographer, when he apparently tried to knock his camera to the ground.

The grand jury is investigating whether Burlingame, California-based BALCO produced illegal steroids or skirted U.S. tax regulations, attorneys familiar with the investigation say. The sports world has been mostly cloaked in secrecy over the case.

The appearance of Marion Jones, winner of a record five Olympic medals in the 2000 Games in Sydney, follows that of her husband, the 100 meters world record holder Tim Montgomery, who testified before the grand jury last week.

The U.S. Anti-Doping Agency has said it suspects that BALCO, which is headed by Victor Conte, provided the previously undetectable THG steroid at the heart of a growing scandal in international sport.

European champion sprinter Dwain Chambers of Britain was suspended earlier this month after testing positive for THG. He has denied knowingly taking any banned substance.

The San Francisco grand jury meets weekly in proceedings expected to last for several months. Last Thursday, Montgomery and sprinter Chryste Gaines, a member of the victorious women's 4x100 meters relay team in the 1996 Atlanta Olympics, testified.

In the coming weeks several football players are expected appear, and on Dec. 4, Barry Bonds, perhaps the greatest active baseball player, and fellow San Francisco Giant Benito Santiago are scheduled to testify.

BALCO says it provided many athletes such as Bonds with blood, urine and hair testing to determine mineral deficiencies. It then suggested nutritional supplements.

The lawyer for Bonds in the case, Mike Rains, told Reuters last week that his client could not rule out the possibility that he might have unknowingly taken steroids hidden in nutritional supplements made by BALCO.

Conte, a former musician, set up BALCO in the 1980s even though he had no background or degrees in science or health. Former associates describe the man at the center of the probe as a quick learner who was able to convince many that he possessed advance insights on nutrition.

"He was very knowledgeable, he was a very smart person," John Bascom, who worked at BALCO from 1994 to 1996, told Reuters. "When he would talk to people, he could spout off things and records, he was very well read about this."

"I think that Conte could out talk anyone and state facts and figures and records and results and stuff, better than anyone I have met in my life."