NEW YORK – George Mitchell, a Washington lawyer and the former Senate majority leader from Maine who previously has advised Major League Baseball, will lead Commissioner Bud Selig’s investigation into alleged steroid use by Barry Bonds and other players.

Under persistent pressure from Congress and in light of a new book that claims Bonds used performance-enhancing drugs for at least five years, Selig will outline his investigation in a news conference Thursday at his office here, baseball sources said.

Mitchell, 72, served on a blue ribbon panel that reported on the financial state of baseball in 2000. He will be the point man in Selig’s first hard look at baseball’s so-called “steroid era,” in which many of the game’s foremost players have been accused of using illegal performance-enhancers. Mitchell is expected to attend today’s news conference.

It is unclear what action Selig would – or could – take if Mitchell and his investigative team found evidence that Bonds and others had taken steroids. The commissioner has appeared resistant to erasing tainted records, and probably would face a legal battle if he employed his “best interests of the game” authority to suspend guilty players.

One, New York Yankee Jason Giambi, reportedly admitted to a federal grand jury that he used steroids, and in a subsequent news conference offered a vague apology. He hasn’t been disciplined because he apparently did not test positive under baseball’s steroid policy. Neither has Bonds.

Major League Baseball did not begin its testing program until 2003, did not suspend its first player until a year ago, and only this year strengthened its policy to sufficiently satisfy Congress.

Certain members of Congress remain unconvinced. In a letter sent to Selig two weeks ago, Rep. Cliff Stearns, R-Fla., cited “apparently credible allegations that [Bonds] used steroids,” and demanded that Selig disclose “your involvement and authority to police against illegal performance-enhancing drug use.”

Stearns, chairman of the House Subcommittee on Commerce, Trade and Consumer Protection, and Rep. Edward Whitfield, R-Ky., chairman of the Committee on Oversight and Investigations, asked Selig to respond to six queries – two directly related to Bonds – by Thursday. According to a source in the commissioner’s office, Selig has received an extension to respond at a later date.

Mitchell will conduct his investigation independently, as when another lawyer, John Dowd, investigated Pete Rose for gambling on baseball in 1989. Dowd submitted a 225-page report, and Rose was banned for life by then-Commissioner A. Bartlett Giamatti.

Twelve major leaguers and 81 minor leaguers were suspended last year under baseball’s previous policy, the major league portion of which suspended first-time offenders for 10 days. The current policy requires a 50-game suspension for a first offense, 100 games for a second offense and a lifetime ban for a third.

“Game of Shadows,” a book written by two San Francisco Chronicle reporters and released last week, claims to detail steroid use by Bonds, Gary Sheffield, Jason Giambi and others, most of whom were alleged to have received designer and anabolic steroids from the Bay Area Laboratory Co-Operative.

The timing of the investigation will be particularly delicate for Bonds and the San Francisco Giants. Bonds is approaching Babe Ruth on the all-time home run list and is 47 short of Hank Aaron’s career record of 755. The Giants open the regular season Monday in San Diego.