That provided the basis for the yearlong bans for former Astros Manager A.J. Hinch and General Manager Jeff Luhnow, both of whom were subsequently fired by team owner Jim Crane. It could also ensnare Alex Cora, who was let go by the Boston Red Sox over his role in the cheating scheme as the Astros’ bench coach. (Cora is awaiting his punishment while M.L.B. investigates the Red Sox for potential illegal use of live video feeds to steal signs during their World Series-winning season in 2018.)

The Yankees had also been particularly wary of the Astros, at least since their World Series victory in 2017. So were several others: Executives from Houston’s rivals were even comparing notes on how they thought the Astros might be cheating. Though the Yankees could not quite figure out how it might be happening, they had been sending any evidence they deemed fishy about the Astros to M.L.B. over the past few seasons.

It took Oakland Athletics pitcher Mike Fiers going on the record with The Athletic in November about his former team’s misdeeds to break the dam and spark M.L.B.’s investigation.

There were a few clues before Fiers went public. In August 2018, the Athletics reported to M.L.B. that the Astros were clapping before pitches, with Oakland believing it was an attempt to relay stolen signs to batters, according to a Yahoo Sports report that year which also mentioned trash can banging.

Earlier that season, during a series at Yankee Stadium, Brandon Taubman, then Houston’s senior director of baseball operations, confronted and threatened a Yankees employee who was filming with a high-tech camera from center field. The Yankees had M.L.B. permission to use the camera since it was nonlive footage for scouting, but Taubman, as part of the Astros’ de facto anti-surveillance team, was playing defense against rivals’ potential sign stealing. (A year later, Taubman was fired by the Astros and banned from M.L.B. for inappropriate comments he made toward female reporters after Game 6 of the 2019 A.L.C.S.)

The Astros’ behavior worried the Yankees enough to prompt extra caution on the diamond. The Yankees instructed their pitchers to frequently change their signs, and gave them cards to place under their caps as reminders of the many variations to cycle through during games. For example: Even with no one on base against the Astros in the 2019 A.L.C.S., Yankees pitchers and catcher Gary Sanchez cycled through different sets of signs.