The Los Angeles Angels have fired their longtime visiting clubhouse manager after an internal investigation confirmed suspicions that he was providing ball-doctoring substances to opposing pitchers, sources confirmed to ESPN on Thursday.

Major League Baseball initially informed the Angels about a week ago about allegations that Brian Harkins, who had been with the Angels for 30 years, was providing illegal sticky substances in the visiting clubhouse that aided pitchers' abilities to grip the baseball, sources with knowledge of the situation said. Harkins confirmed that he had been let go Thursday, but did not elaborate on the reason. The Angels did not comment, either, citing legal restrictions preventing them from discussing employee matters.

The Los Angeles Times first reported the news.

Angels visiting clubhouse manager Brian "Bubba" Harkins, right, has been fired by the team for providing ball-doctoring substances to opposing pitchers, sources confirmed to ESPN on Thursday. John Cordes/Icon Sportswire

Pitchers have been using foreign substances to doctor baseballs throughout the game's history, an illegal practice that was nonetheless accepted largely because of its widespread use. Cincinnati Reds pitcher Trevor Bauer recently told Real Sports that 70% of pitchers use some sort of substance -- be it pine tar or Firm Grip or a homemade concoction -- to enhance their grip, which could ultimately provide greater movement and velocity.

MLB intends to strongly enforce the rule that guards against the practice this season and has made teams aware during spring training. The Angels were not believed to have been pressured by MLB to fire Harkins, but the league's proactive approach in this matter could be an early sign of its intent to follow through. Harkins provided pitchers with a substance that consisted of a melted-down pine-tar solution and rosin, a source said. The Angels believe Harkins acted alone, and they did not fire any other clubhouse attendants for this matter.

Harkins, known throughout the game as "Bubba," joined the Angels as a bat boy in 1981 and returned to the organization as a clubhouse manager in 1986, taking over the visiting clubhouse four years later. He was named visiting clubhouse manager of the year by MLB equipment managers in 2005.

ESPN's Jeff Passan contributed to this report.