In August 2017, the Yankees asked the Commissioner's Office to investigate "the illegal use of electronic equipment" by the Red Sox in order to steal signs and relay the information to batters. After that investigation, Rob Manfred sent the letter quoted below to Yankees GM Brian Cashman on September 14, 2017.
The Yankees fought for years against the public release of this letter. Now we can see why. Evan Drellich (The Athletic) notes that Manfred's letter reveals the stark difference between the way the Yankees' behavior was described publicly and privately.
In his 2017 statement, Manfred ruled the Red Sox had violated the rule against using electronic equipment "for the purpose of stealing signs or conveying information".