Major League Baseball and the Houston Astros are investigating claims the team illegally used a camera to steal signs from catchers to pitchers during their 2017 World Series-winning season.
Former Astros pitcher Mike Fiers and three anonymous sources told The Athletic that Houston used a camera in the outfield during home games.
MLB prohibits the use of electronic equipment to steal signs.
But the practice, while frowned upon, is otherwise not against the rules.
Catchers flash signs to pitchers using their fingers to call for a specific delivery - such as a fastball, curveball or slider - and knowing what pitch is likely to be coming is an advantage to the batter.
The Athletic report claims that during the 2017 season, Astros players and staff would monitor opposing catchers' signals as recorded by a camera at Minute Maid Park.
After working out which sign corresponded to which pitch, they would tell the batter what was coming, allegedly banging on a rubbish bin inside the clubhouse to signal an off-speed pitch.
Players are permitted to steal signs in the field, for example when a runner on second base is able to see what signal the catcher is giving, though it is often seen as gamesmanship.
"The Houston Astros organisation has begun an investigation in cooperation with Major League Baseball," the Astros said in a statement on Tuesday.
"It would not be appropriate to comment further on this matter at this time."
The Astros beat the Los Angeles Dodgers in game seven, at Dodgers Stadium, to win a thrilling 2017 World Series.
It was their first and only championship title, with Houston having been beaten 4-3 by the Washington Nationals in this year's World Series.
Fiers, 34, was not offered a new contract in Houston after the end of the 2017 season, having been left off their roster for the postseason. He moved to the Detroit Tigers for 2018, and is currently with the Oakland Athletics.
"I just want the game to be cleaned up a little bit because there are guys who are losing their jobs because they're going in there not knowing," Fiers said.
"Young guys getting hit around in the first couple of innings starting a game, and then they get sent down [to the minor leagues].
"The guys who know are more prepared. But most people don't."