SAN FRANCISCO -- Three track and field athletes who flunked drug tests have been subpoenaed to testify before a federal grand jury that is investigating whether a prominent San Francisco nutritionist has links to what one anti-doping official calls "an international doping conspiracy," ESPN has learned.

The three athletes were tested at the U.S. Track & Field Nationals in Palo Alto, Calif., in June. The results were reported in the last several weeks. One of the athletes was a client of BALCO Labs, the supplement and nutrition company run by Victor Conte. The controversial nutritionist boasts of a roster of professional and Olympic sports stars as customers.

As many as 40 other athletes also may have received subpoenas, according to a source close to the case, who added: "The names I've heard are some of the biggest names in sports."

"I know of no other drug bust that is larger than this involving the number of athletes involved," anti-doping official Terry Madden, director of the U.S Anti-Doping Agency, told The Associated Press. He refused to reveal the names or genders of the athletes, or to be more specific about how many had tested positive.

On Thursday, Madden identified Conte as the alleged supplier of the previously undetected steroid THG, which was detected in the three athletes -- a charge Conte denied.

"What we have uncovered appears to be intentional doping of the worst sort," Madden said during a telephone news conference. He called the case "a conspiracy involving chemists, coaches and certain athletes to defraud their competitors, and the American and world public who pay to attend sporting events."

THG was discovered in early May, when a source whom Madden would identify only as a "high-profile" track coach called USADA, an independent agency charged with handling drug cases for the U.S. Olympic Committee.

Although the coach refused to identify himself, he offered to send USADA a syringe filled with the drug that he said was being used by cheaters, according to Madden. The syringe was delivered to the Olympic Analysis Lab at UCLA, which has made several high-profile discoveries of underground designer steroids in the last two years. The lab's director, Don Catlin, ultimately identified the substance as tetrahydrogestrinone, a relative of the banned steroid trenbolone.

Until then, USADA hadn't tested for THG because it didn't know the drug existed, Madden said.

Madden described THG as a "very sophisticated designer steroid created by some very sophisticated chemists" who are under federal investigation. He said the athletes placed a few drops of the oil-based steroid under their tongues and expected that any trace of it would pass through their bodies quickly.

But the drug did not pass as quickly as they apparently thought. Catlin developed a test for the steroid in secret, then used it to retest samples taken from the athletes at the Nationals. About 350 tests were conducted on competitors at that event. A hundred more tests were done in other sports.

Madden would not identify which sports were targeted, or say how many athletes have tested positive. He also would not comment on the investigation that led USADA officials to identify Conte publicly. He did say, however, that the ongoing inquiry into the use of THG has expanded to include American professional sports leagues.

NFL spokesman Greg Aiello said Friday he could not comment on whether the NFL was involved in the USADA probe, but said the league will soon include THG in its steroid tests.

"Is this something we are or will be testing for? The answer is yes," Aiello said in a telephone interview on Friday. "This is clearly a new type of steroid and we will be testing for it. Nobody was aware of it until recently."

"This is a serious warning for cheaters," Dick Pound, chairman of the World Anti-Doping Agency, told AP on Thursday. "It shows that supposedly undetectable substances can be detected as new tests are developed."

In September, agents for the Internal Revenue Service raided BALCO, in Burlingame, Calif., carting out boxes of records and raising suspicions that federal agents are interested in its finances. By then, Madden said, they already had been told about USADA's findings. BALCO's clients include Barry Bonds, Bill Romanowksi, and Olympic stars Marion Jones and Tim Montgomery.

Sprinter Kelli White, who flunked the test for the stimulant modafinil at the recent world track and field championships, also is associated with BALCO, and has been subpoenaed to testify before the federal grand jury, according to The San Francisco Chronicle. Her case is being considered by USADA, and could cost her a pair of gold medals. That drug, a stimulant, has no connection with THG.

Madden said that his staff has not contacted Conte. Don Clay, the assistant U.S. attorney in San Francisco who is heading up the case, did not return calls seeking comment.

In e-mails to several newspapers Thursday, Conte denied BALCO was the source of the substance.

But Conte, in an e-mail Thursday to The Associated Press and other news organizations, said BALCO was not the source of the substance.

"In my opinion, this is about jealous competitive coaches and athletes that all have a history of promoting and using performance-enhancing agents being completely hypocritical in their actions," he said.

In another e-mail Friday to the AP, Conte wrote that the USADA was wrong to compare THG to anabolic steroids.

"In short, my opinion is that this case is more about politics than science," he said. "There is absolutely zero evidence that this substance has any anabolic effects."

Conte, a former bassist for the group Tower of Power, has been a nutritional consultant in the Bay area since the mid-1980s. He is widely known for touting the use of zinc to boost testosterone production, and monitoring athletes' mineral levels through blood and urine tests.

In the June issue of Muscle & Fitness magazine, Bonds enthused about Conte's physical fitness regimen and nutritional advice, saying, "I'm just shocked by what they've been able to do for me."

Bonds' agent, Scott Boras, told The San Francisco Chronicle this week that the investigation "really doesn't involve Bonds."

Shaun Assael is a senior writer for ESPN the Magazine; he can be reached at shaun.assael@espn3.com. Information from The Associated Press was used in this report.