Major League Baseball is investigating claims that the Boston Red Sox used video replays to steal signs from catchers to pitchers during their 2018 World Series-winning season.
A report in The Athletic claims players visited Boston's replay room, relaying information to team-mates on the field.
Catchers use hand signals to tell pitchers what delivery to throw, such as a fastball or curveball.
The use of electronic equipment to steal signs is banned by MLB.
The practice, while frowned upon, is otherwise not explicitly against the rules.
The Red Sox say they will "fully cooperate" with the investigation, which relates to the 2018 regular season when they won a league-high 108 games.
The claims are the second accusations of sign stealing in two months after MLB opened an investigation into the Houston Astros in November.
The Athletic had earlier alleged that the Astros illegally used a camera in the outfield to steal signs during their 2017 World Series-winning season.
In September 2017, the Red Sox were fined by MLB for using an Apple Watch to steal signs from the New York Yankees.
After that incident, MLB issued a specific warning to all 30 teams over technology.
Following the latest claims, MLB said: "The commissioner made clear in a 15 September 2017 memorandum to clubs how seriously he would take any future violation of the regulations regarding use of electronic equipment or the inappropriate use of the video replay room.
"Given these allegations, MLB will commence an investigation into this matter."