SAN FRANCISCO -- Fourteen months after starring in history's fastest race, Tim Montgomery and Dwain Chambers again are linked in a headline-grabbing event.

This time, though, the drama is taking place far from the track. It is based in courtrooms and laboratories, where grand jurors and scientists are sorting through the sporting world's latest drug scandal.

Montgomery, the world's fastest man, testified Thursday before a federal grand jury in San Francisco that sources say is probing drug use by some of the nation's top athletes as well as possible wrongdoing by a Bay Area lab.

Chambers blames that same lab for his positive drug test for the newly discovered steroid THG. Chambers was suspended Friday after UK Athletics confirmed he had tested positive for the steroid, and now faces a two-year suspension and a lifetime ban from the Olympics.

It was in September 2002 that Montgomery set his 100-meter world record of 9.78 seconds at a meet in Paris. Chambers finished second in 9.87, matching the European record.

On Thursday, Montgomery was among the latest U.S. track and field stars to appear before the grand jury. Sprinter Chryste Gaines, a 1996 Olympic relay gold medalist, also went before the panel Thursday, a week after shot putter Kevin Toth and 1,500-meter runner Regina Jacobs were among those testifying.

Dozens of other athletes, including baseball's Barry Bonds and Jason Giambi and several NFL players, also have been subpoenaed in the case. Boxer Shane Mosley has been subpoenaed as well.

All have been customers of the Bay Area Laboratory Co-Operative, or BALCO. An attorney for BALCO founder Victor Conte has said his client is the target of the grand jury probe.

Montgomery, 28, who is the boyfriend of Olympic sprinting champion Marion Jones, did not comment after his morning appearance. His agent, Charlie Wells, who came with Montgomery to the grand jury session, also declined to comment.

Gaines, 33, who ran the two fastest 100 times of her career in the last few months, declined to comment after her afternoon appearance.

It's not clear what, if any, drug charges might result from the investigation. An appearance before the grand jury, or being subpoenaed to testify, does not mean an athlete is a target of the probe.

Sources familiar with the investigation have said those athletes being called to testify are granted immunity from prosecution for any wrongdoing they may acknowledge before the grand jury.

Federal officials have refused to discuss the grand jury or the scope of its secret proceedings, but two sources familiar with the grand jury have said the probe is focusing on drug use by athletes as well as possible tax evasion by BALCO.

BALCO also is at the center of an investigation by anti-doping agencies into THG. An unidentified coach who turned in a used syringe containing THG said he got the substance from Conte.

At least five athletes already have tested positive for THG, including Jacobs, Toth and Chambers.

"They're looking at at least five different drugs -- steroids, hormones, THG, modafinil, EPO," said one source close to the grand jury probe who asked that he not be identified.

Another source close to the grand jury proceedings said one subpoenaed athlete claims Conte promised THG would go undetected in urine samples provided to drug testers. The athlete knowingly used THG, the source said.

The source said Conte repeatedly tested the athlete's urine to confirm THG would not be discovered in doping tests, and the athlete rewarded Conte with $10,000 from winnings at a competition this year.

The source spoke on condition of anonymity, and demanded that the athlete not be identified either.

Conte declined comment Thursday.

Douglas Schwartz, an attorney representing Jacobs, Gaines and other athletes scheduled to appear before the grand jury, said the athletes being subpoenaed are not targets of the probe.

"The Department of Justice policy is that targets of a grand jury do not receive subpoenas. If you receive a grand jury subpoena then, at least at that time, you are not a target." he said.

Schwartz also contended athletes did not break any laws if they used THG. The U.S. Food and Drug Administration declared THG an illegal drug late last month. Prosecutors and lawyers disagree whether it was illegal to take THG before then.

"I think you will find any athlete that used THG did so without knowledge of what it was," Schwartz said. "But with or without that knowledge, it is not a controlled substance."

Chambers' backup B sample was analyzed this week at UCLA, and UK Athletics said Friday it was notified by the sport's world governing body that analysis of the backup sample matched the positive A test.

Chambers, considered a medal contender for next summer's Athens Olympics, was suspended pending a British disciplinary hearing. He faces a minimum suspension of two years and a lifetime Olympic ban.

Chambers denied knowingly taking a banned substance and blamed the positive test on nutritional supplements supplied by BALCO.