SAN FRANCISCO -- The federal prosecutor who has led the high-profile probe into athlete steroid abuse announced he was stepping down, and critics alleged political pressure from the Bush administration was pushing him and others out of their jobs.

Kevin Ryan, U.S. attorney for California's Northern District, has headed the investigation into the Bay Area Laboratory Co-Operative, the Burlingame supplements lab at the center of the doping scandal that has touched some of professional sports' biggest stars.

The BALCO investigation has led to five guilty pleas, spurred congressional hearings and cast suspicion on San Francisco Giants slugger Barry Bonds. A federal grand jury is probing whether Bonds lied to an earlier grand jury about whether he knowingly used performance-enhancing substances.

Two San Francisco Chronicle reporters are facing jail for refusing to tell prosecutors who leaked secret BALCO grand jury testimony to them.

Ryan is one of 11 top federal prosecutors who have resigned or announced their resignations since an obscure provision in the USA Patriot Act reauthorization last year enabled the U.S. attorney general to appoint replacements without Senate confirmation. Carol Lam, who headed California's Southern District, also announced Tuesday she would be leaving.

California Sen. Dianne Feinstein, a Democrat, complained on the Senate floor Tuesday that the White House is using the provision to oust Ryan, Lam and other federal prosecutors and replace them with Republican allies.

"The Bush administration is pushing out U.S. attorneys from across the country under the cloak of secrecy and then appointing indefinite replacements," Feinstein said.

Attorney General Alberto Gonzales denied the claim, saying administration officials "in no way politicize these decisions."

Ryan, who was appointed to his post in 2002, replacing now-FBI director Robert Mueller, reached a "mutually agreeable decision with Washington" to step down, spokesman Luke Macaulay said. He declined to say whether President Bush had asked Ryan to resign.

Ryan did not give a date for his last day on the job.

During his tenure, Ryan also oversaw prosecutions on stock options fraud, with at least 106 companies facing scrutiny from the Department of Justice or the Securities and Exchange Commission on questions about the timing of options grants. Options give recipients the right to buy stock at a fixed price, generally the stocks' market price the day of the grant.

The inquiries are studying whether grants were "backdated" to coincide with low stock prices, maximizing the holders' potential for profit.

Criticism of Ryan mostly has focused on his administration of the office. Last year, a Justice Department audit reportedly questioned Ryan's management and rated staff morale as low.