One of baseball’s oldest rivalries is embroiled in a most 21st-century controversy, as Major League Baseball prepares to discipline the Boston Red Sox for using an Apple Watch to steal signs from their longtime enemies, the New York Yankees.

MLB Commissioner Rob Manfred confirmed Tuesday night that the Red Sox have been "100% cooperative" with the league's investigation, which first came to light in a New York Times report, and noted that there is no rule explicitly barring sign-stealing.

However, the Red Sox would be in violation of baseball regulations banning the use of certain electronic devices in the dugout.

A baseball official with direct knowledge of the investigation told USA TODAY Sports the the probe also includes the unauthorized use of cellular phones and noted that the practice also occurred against other teams. It involves at least four Red Sox players.

The person spoke to USA TODAY Sports on condition of anonymity due to the ongoing investigation.

While both Manfred and Red Sox President of Baseball Operations Dave Dombrowski noted that sign-stealing gone too far is an issue often handled from one general manager to the next, the Red Sox's means to pass signs along to their batters goes far beyond any written or unwritten baseball statutes.

The Times reported that the Red Sox used an elaborate relay system – involving video replay personnel, a trainer and players in the dugout - to pass along signs from the opposing team to their own batters. While the report cited “other teams” from which the Red Sox stole signs, it primarily occurred against the Yankees.

Klapisch:Yankees thriving on heat and home runs

Power rankings:Indians streaking, Dodgers still No. 1

The system was uncovered by the Yankees themselves, who videotaped the process during a three-game series at Fenway Park last month. The Times reported that the video provided to the commissioner’s office shows Red Sox assistant athletic trainer Jon Jochim looking at his Apple Watch, then passing along a signal to outfielder Brock Holt and second baseman Dustin Pedroia, who was on the disabled list at the time.

The video then purportedly shows Pedroia passing the information to teammate Chris Young on second base, who then relays it to the batter.

The longtime – and more low-technology – method of sign-stealing has long been for the runner on second to pass signs on to the batter, a practice that’s not banned so long as artificial means are not used. (Although the practice certainly can create tension between the two teams).

"It's always been a game within a game, but the use of electronics takes it too far," Yankees outfielder Brett Gardner told reporters on Tuesday.

The Red Sox filed a cross-complaint with MLB, alleging that the Yankees use YES Network cameras at Yankee Stadium to steal signs during games.

Tuesday, Yankees manager Joe Girardi denied that allegation, telling reporters that there's "no chance. "We're not doing it."

At a Fenway Park news conference, Red Sox President of Baseball Operations Dave Dombrowski refused to address specifics of the accusations, noting that sign-stealing has long been a part of the game.

MLB approved the use of tablets in the dugout - and reached a multi-year agreement with Apple to ensure the iPad Pro was the tablet of choice - before the 2016 season. In announcing the deal at the time, Commissioner Rob Manfred said that deal "is part of our ongoing effort to introduce extraordinary technology into our game. We are pleased that Apple’s groundbreaking products, which have already improved the access that fans have to our sport, will now inform the decisions that make our games interesting and exciting throughout the year.”

Yet the dugout bullpen devices do not have Internet connectivity or the ability to stream live video - to prevent the sort of activities for which MLB is investigating the Red Sox.

And were it not for the advent of instant replay, the Red Sox would not have such immediate access to camera angles at better vantages than the usual runner at second base.

During the series in question, the Times reports, the Red Sox went 5 for 8 with runners on second base, including a Rafael Devers home run. They went 1 for 6 and 3 for 10 in subsequent games.

The Red Sox entered Tuesday night’s games 2 ½ games ahead of the Yankees in the AL East, and at 77-61 have the third-best record in the AL. They are 40-26 in games played at Fenway Park, and 37-35 on the road.

Gallery: MLB photo of the day