SAN FRANCISCO – The Bush administration has opened a new front in its war on leaks with an aggressive and unusual move to force two San Francisco Chronicle reporters to identify their sources for stories about secret grand jury testimony from a federal investigation of steroid use in professional baseball.

Federal prosecutors served subpoenas Friday on the newspaper and the journalists, Mark Fainaru-Wada and Lance Williams, who immediately vowed to resist on First Amendment grounds.

The legal confrontation could unfold quickly, as both men have been ordered to appear at the federal courthouse here Thursday morning to testify before a separate grand jury investigating the leaks.

“It’s a further escalation really of the war that the Bush administration has been waging on the press and public’s right to know,” a journalism professor at George Washington University, Mark Feldstein, said in an interview yesterday.

While federal prosecutors have recently become increasingly aggressive in pursuing leaks and last year caused the jailing of New York Times reporter Judith Miller for refusing to identify her sources in a CIA-related leak case, the subpoenaing of the Chronicle reporters suggests that the Justice Department is willing to use tough tactics even in leak cases that have nothing to do with national security.

“I’m not aware of an ordinary leak case resulting in subpoenas of reporters. It may have happened but I’m not aware of it,” a top Justice Department official in the Clinton Administration, Jamie Gorelick, said.

“What we’ve seen in the last couple of years in the Bush Administration, I’m sorry to say, is a zero-tolerance policy for leaks and a willingness to break a lot of china to plug them,” Mr. Feldstein said. He said the actions in the steroid case are consistent with a visit he recently received from FBI agents wanting to look for classified documents in the files of a prominent syndicated columnist who died last year, Jack Anderson.

“National security is now morphing into steroid use by athletes. That’s a hell of a lot different and I think a much flimsier reason to engage in such encroachment,” the journalism professor said.

Legal observers said the drive to unmask the sources of the San Francisco reporters is certain to have the approval of senior Justice Department officials in Washington. Under department regulations, the permission of the attorney general is required before subpoenas may be issued to members of the press.

The subpoenas to the Chronicle and its reporters focus on articles published in June and December 2004 quoting grand jury testimony by prominent athletes, including two baseball sluggers, Jason Giambi of the Yankees and Barry Bonds of the San Francisco Giants, as well as a sprinter, Timothy Montgomery.

The stories said Mr. Giambi admitted using steroids and injecting himself with human growth hormone. Mr. Bonds allegedly acknowledged using substances produced by a Burlingame, Calif. laboratory at the center of the probe, the Bay Area Laboratory Cooperative, or Balco.

According to the newspaper, Mr. Bonds said he did not know the compounds were steroids. In his testimony, Mr. Montgomery conceded he used a “magic potion” from the lab, but said he was assured it was not a steroid.

No athlete has been charged publicly with a crime in the steroid investigation, although several people affiliated with the lab have pleaded guilty to federal offenses.

The stories contained what appeared to be direct, verbatim quotations from the athletes’ grand jury testimony and included questions asked by prosecutors. The paper has not said how it obtained the information. The stories do not make clear whether the journalists had the transcripts in their possession or were granted temporary access to them.

It is illegal for prosecutors, investigators, court personnel, and grand jurors to disclose grand jury testimony. However, witnesses are generally free to discuss what transpired during their appearances.

The subpoenas, which were released by the newspaper, focus squarely on the leaked testimony, seeking “any information in your possession regarding the identity of the individual or individuals who provided the Chronicle a copy of any grand jury transcript” related to the case.

The subpoenas also seek details about “who disclosed information about the content of any such witness’s grand jury testimony,” as well as the original packaging in which the journalists or the newspaper received any of the secret transcripts.

Earlier this year, Messrs. Fainaru-Wada and Williams published a book about the steroid investigation, “Game of Shadows.” The subpoenas make no reference to the new work.

The leak probe is being overseen by the United States attorney in Los Angeles, Debra Wong Yang, apparently because lawyers and investigators for the federal prosecutor in San Francisco had access to the secret transcripts and could have been sources for the press leaks. A spokesman for Ms. Yang declined to comment yesterday.

In earlier court proceedings, the San Francisco prosecutors have vehemently denied providing the information that appeared in the Chronicle stories, as have defense attorneys.

Mr. Bonds’s legal team, which has been pushing for an aggressive inquiry into the leaks and even asked that the reporters be forced to surrender their earnings from the book, welcomed the development. “We think it’s about time,” a lawyer for Mr. Bonds, Alison Berry Wilkinson, told The New York Sun yesterday.

Ms. Wilkinson said the leaks should be investigated thoroughly even though there is no national security angle to the steroid probe,which is continuing with a reported focus on whether Mr. Bonds testified truthfully.

“Perhaps there are no national defense implications, but this is a very significant case,” she said. “How can anyone trust, with this most recent grand jury inquiry, that information provided won’t be splashed across the front pages? People have a disincentive to cooperate. It’s harming the investigation.”

In its pages, the Chronicle reacted angrily to the new subpoenas. An editorial yesterday described them as part of a “war on the press” mounted by the Bush administration.

“The San Francisco Chronicle unconditionally stands by its reporters in fighting this effort by the government to force them to reveal their confidential sources,” the newspaper’s executive editor, Phil Bronstein, said. “Our reporters broke no laws, nor is the government accusing them of having done so.”

“We believe that our ability to report on this issue is protected by the First Amendment to the Constitution. In the case of the steroid story, the public was well served by that reporting, which sparked a national discussion and debate about steroid use in sports and ultimately led to enactment of stricter steroid policies by Major League Baseball,” the editor said.

A Justice Department official in the administrations of Presidents Reagan and George H.W. Bush, Douglas Kmiec, said officials may have felt compelled to go after the Chronicle because of the flagrant leak of verbatim transcripts. “To try and stop the substantial amount of information that does appear generally in the press in garden variety cases would easily overtax the capability of the department,” he said.

A leading legal advocate for journalists, Jane Kirtley, said yesterday that the steroid-related subpoenas do signal an increased willingness by federal prosecutors to seek testimony from the Fourth Estate. “I do think there has been a shift and I think it’s based in part on the fact that some pretty influential federal courts have shown they’re not averse to subpoenaing the press,” said Ms. Kirtley, a law professor at the University of Minnesota and a former head of the Reporters Committee for Freedom of the Press.

In the case involving Times reporter Judith Miller, a federal appeals court in Washington ruled she had to testify about her conversations with Bush administration officials about the identity of a CIA operative.

The Supreme Court refused to take Ms. Miller’s case, but Ms. Kirtley said it’s possible the steroid-related dispute could end up before the justices, especially if the 9th Circuit Court of Appeals, which is often more liberal, rules in favor of the journalists.