SAN FRANCISCO -- The federal agent whose hunches helped spark the government's probe of Barry Bonds and the BALCO steroids-distribution ring testified publicly for the first time Thursday about his detective work -- from picking through trash to analyzing bank databases.

Jeff Novitzky, an IRS special agent, dangled hints about sports figures touched by the federal government's investigation of performance-enhancing drugs in sports, but at the direction of prosecutors, identified none. On eight occasions, Assistant U.S. Attorney Jeff Nedrow directed Novitzky to describe his investigation without naming names.

The testimony came on the fourth day of the perjury and obstruction of justice trial of cyclist Tammy Thomas, who's charged with lying to a grand jury, thereby hampering the Bay Area Laboratory Co-Operative investigation.

Barry Bonds' legal team is expected to monitor the trial closely, because the home run king faces nearly identical charges.

Novitzky described surveillance in 2003 of elite athletes coming and going from BALCO, the Burlingame business at the scandal's center. One unidentified person went from BALCO "directly to Pacific Bell Park, I think it was called at the time," referring to the stadium where the San Francisco Giants play, now called AT&T Park.

"The majority of the athletes were from the sport of track and field, but there were athletes from baseball, football," ranging from well-known to obscure athletes, Novitzky said. Some of their jerseys hung on the wall of BALCO's gym, he said.

Novitzky, testifying as part of the prosecution's case, looked directly at the jury of San Francisco Bay area residents and told them he, too, is from this region. He said he became aware of BALCO founder Victor Conte in the late 1980s as Conte's nutritional supplements gained visibility.

Years later, Novitzky noted Conte defended track star C.J. Hunter after Hunter tested positive for doping. "It sparked my interest," Novitzky said.

In 2002, Novitzky said he began researching Conte and found Internet postings by Conte with "intricate knowledge of performance-enhancing drugs," including how much they cost and what they could do for athletes.

The special agent ran Conte's name, and BALCO's, through a government database that tracks transactions larger than $10,000 and found a $14,000 cash withdrawal, further piquing his curiosity. A grand jury subpoenaed other financial records, he said.

Novitzky began searching through the trash behind the BALCO offices, learning when the company set garbage out and when it was collected. Each Monday night for a year, he hauled BALCO's rubbish to a well-lit area nearby and sifted through it, he testified.

He found copies of e-mail messages and copious quantities of empty needle wrappers, he said. The latter led him to a medical-waste company where he found evidence of syringes, vials and performance-enhancing drugs that apparently originated at BALCO.

One of the "trash runs," as he put it, yielded evidence that BALCO was buying epitestosterone from a company in the Midwest, he said.

Novitzky also obtained warrants to search e-mail traffic related to the case and became interested in Thomas when one e-mail from her mentioned Conte. She also had tested positive for norbolethone, an obscure steroid, Novitzky testified.

Thomas' e-mail was sent to chemist Patrick Arnold. In Thomas, "we thought we had someone with potential direct knowledge of one of the manufacturers of one of these substances, which would be very valuable to us," he said.

Prosecutors gave Thomas immunity to testify in October 2003. But her statements to the grand jury were "inconsistent," and she claimed she had not gotten the steroids from Arnold, Novitzky said.

Her failure to fully cooperate, he said, set back the investigation and Arnold's indictment, he said.

Eight people connected to BALCO, including Arnold, have pleaded guilty to drug charges or perjury. Thomas is the first to go to trial.

An unusually large contingent of lawyers showed up to monitor the star witness' testimony Thursday afternoon, many scribbling notes on legal pads.

Novitzky is scheduled to take the witness stand again Monday, when he will be cross-examined by defense lawyers.