NEW YORK -- Former Sen. George Mitchell reportedly is seeking cooperation from 45 current or former players as he continues his investigation into the use of performance-enhancing drugs in Major League Baseball.

According to The New York Times, the number of players was disclosed in a letter from the Major League Baseball Players' Association to Mitchell and was read aloud to one of its reporters by a representative of the MLBPA.

The letter, which was signed by MLBPA general counsel Michael Weiner, voiced concerns about players cooperating with the investigation.

The New York Yankees' Jason Giambi is the only active player known to have cooperated with the Mitchell investigation, which is hampered by the lack of subpoena power.

MLB commissioner Bud Selig selected Mitchell to pilot the investigation, which was formed in the wake of the book "Game of Shadows," which illustrated detailed steroid use by San Francisco Giants slugger Barry Bonds and others.

According to the book, written by reporters Mark Fainaru-Wada and Lance Williams of the San Francisco Chronicle, Bonds began using steroids after the 1998 season because of his jealousy stemming from the attention that Mark McGwire and Sammy Sosa garnered during their chase, and eventual passing, of Roger Maris' single-season home run mark.

Bonds eventually broke McGwire's mark of 70 after he belted 73 homers during the 2001 season. He has continuously denied knowingly taking steroids. Meanwhile, Major League Baseball has been constantly battling the alleged use of steroids for some time now.

Mitchell, a senator from Maine from 1980-95 and the Senate Majority Leader from 1989-95, has had extensive investigative experience, starting with his time as a United States attorney. He also headed the investigation into the improper bidding process for the Salt Lake City Winter Olympics in 2002.