"This will take as long as it needs to," said Mitchell, who is head of a team that includes attorneys Jeffery Collins and Thomas Carlucci, who were also in attendance.
Mitchell is a director of the Boston Red Sox, as well as chairman of The Walt Disney Co., the parent of ESPN.
Mitchell said his role with the Red Sox will not create a conflict. "If, in any way, anyone associated with the Red Sox is implicated, they will be treated just like everyone else," he said.
The probe was spurred by recent allegations made in a book that targets San Francisco Giants slugger Barry Bonds and Gary Sheffield and Jason Giambi of the New York Yankees, among a number of other former Major Leaguers.
But the Commissioner was careful not to mention the names of any specific players during the briefing.
"The speculation was originally fueled by the testimony of players before a federal grand jury investigating into the Bay Area Laboratory Co-Operative (BALCO) and by an alleged relationship between certain players and BALCO defendant, Greg Anderson," Selig said, referring to Bonds' former personal trainer, who was indicted along with three others by the grand jury. "A recent book has amplified the allegations and raises ethical issues that must be confronted head on."
The book, entitled "Game of Shadows," alleges that Bonds used performance-enhancing drugs beginning in 1998 for a five-year period, which includes his record-breaking 73-home run, 2001 season. It also said that Giambi was an experienced long-time user and that Sheffield extensively was involved in taking the drugs after he came under Anderson's wing.
Selig said on Thursday that the initial scope of the investigation would involve whether any of the BALCO-related players took any illicit drugs since testing for performance-enhancing substances commenced with the signing of the 2002 Basic Agreement.
"I have asked Sen. Mitchell to attempt to determine, as a factual matter, whether any Major League players associated with BALCO or otherwise used steroids or other illegal performance-enhancing substances at any point after the substances were banned by the 2002-2006 Collective Bargaining Agreement," Selig said. "It may be that conduct before the effective date of the 2002 Basic Agreement will be helpful in reaching the necessary factual determinations. If the Senator so concludes, he will investigate such earlier conduct as well."
Moreover, Bob DuPuy, MLB's president and chief operating officer, emphasized that the scope of the investigation could become quite broad.
"That's far too narrow," DuPuy said. "The Commissioner wants the panel to look at it and take it as far as they want to go."
Bonds has been the focal point of the controversy since his leaked grand jury testimony of 2003 linked him to steroid use. Bonds will resume his chase of the all-time career homer record when the Giants open the season against the Padres in San Diego on Monday. Bonds, at 708, is six homers in arrears of Babe Ruth and 47 behind Hank Aaron, the all-time leader with 755.
Selig said that the timing of the investigation had nothing to do with the fact that Bonds is nearing one of baseball's most cherished records.
"It wasn't even taken into consideration," he said, adding that MLB is in the process of making plans to honor Bonds when and if he passes Ruth sometime during the season's opening month.
Asked about the investigation, Bonds twice declined to comment to reporters on Thursday.
All the players involved will be allowed to participate with their teams while the probe is under way, DuPuy said.
"I expect them to play the whole season," DuPuy said. "I expect Bonds to play the whole season, his physical limitations not withstanding."
Selig said that no matter what the outcome, the results of the investigation will be made public.
"You will get the information as soon as we're done with it," he said. "That's the point of the investigation."
Though the panel doesn't have legal power to compel players to cooperate with the investigation, Giambi, Sheffield and the Giants said on Thursday that they are all prepared to help, if needed.
"I have no problem doing anything," said Giambi, last season's Comeback Player of the Year Award winner in the American League. "I did the things I needed to do, took care of them, played last year and have gone forward. I'm not really worried about it."
"If I have to, I will," Sheffield said. "I'm going to do whatever the law [requires] me to do. Other than that, it's a waste of my time."
The Giants released a statement that read: "As we have said all along, the Giants support the Commissioner's decision to review all aspects of what is obviously a serious matter. We commend the selection of George Mitchell to head up this investigation and we pledge our full support to the Commissioner, Senator Mitchell and his team."
MLB did not have random testing for steroids and other performance-enhancing drugs prior to 2003, though Selig circulated memos during the 1990s stating that the use of those drugs by players was strictly prohibited and could be cause for discipline. The players association declined to collectively bargain the issue at the time.
Don Fehr, the executive director of the union, said on Monday that under those terms, players could only be tested and penalized when MLB officials had "just cause" to believe any Major Leaguer was doing such drugs. MLB never announced player penalties then and has not reported that either Bonds, Giambi or Sheffield have failed a drug test since the twice re-written MLB drug policy went into effect four years ago.
Selig said it was far too early in the process to speak about what disciplinary action he might take in the wake of the investigation. The union, which has been fully informed about the probe, has a representational obligation to any of the players involved, Fehr said.
"I hope nobody is making judgments about the inquiry before it's done," said Fehr, who met with Bonds at his Scottsdale Stadium locker for 20 minutes on Monday after the union's annual spring session with the Giants players. "Bud will make whatever decision Bud makes and we'll go from there."
The book, which was written by a pair of San Francisco Chronicle reporters who covered the federal investigation into BALCO, says Bonds used a host of steroid-based drugs to improve his strength, play and recovery time from injuries.
Bonds missed all but 14 games of the 2005 season after having surgery three times on his right knee and returned on Sept. 12 to hit five homers in his first 36 at-bats.
Bonds has hit four more this spring in 17 at-bats and is batting .588. Bonds had not played since Friday because of inflammation in his left elbow, but he was back in the lineup as the designated hitter Thursday night when the Giants lost a rain-shortened exhibition game, 1-0, to the Los Angeles Angels of Anaheim at San Francisco's AT&T Park. He walked once in two plate appearances.
Earlier this month, when excerpts of the book were published in Sports Illustrated, Selig said he would review all the pertinent information pertaining to Bonds' alleged steroid use. The Commissioner has been under increased pressure from Congressional leaders and the media to open an investigation into just what happened in the Major Leagues during the alleged steroid era, which began in 1998 when Mark McGwire and Sammy Sosa chased and broke Roger Maris' single-season, 37-year-old home run record of 61.
Selig said that despite the revelations of the BALCO investigation and all the accruing publicity during the last three years, now was the time to open the investigation.
"The book just goes into much more detail," he said. "I am very troubled by the alleged depth of the relationship between certain players and those involved in the illegal distribution of performance-enhancing substances."