The NFL is investigating the Giants’ illegal use of a walkie-talkie to communicate with quarterback Eli Manning from the sideline in Sunday’s game against the Cowboys, league sources have confirmed.

In the fourth quarter of the game, the usual system used to relay plays from Ben McAdoo to a radio in Manning’s helmet malfunctioned. That sent the Giants scurrying for ways to get the calls to the field. At times Manning would walk to the sideline and at other times Odell Beckham Jr. would get the call and bring it back to the huddle.

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The Giants eventually figured out that a walkie-talkie could be used to get the plays in and McAdoo used it to call the plays on that series, which ended with Manning throwing an interception.

One of the sources told Newsday that league policy prohibits coaches from using walkie-talkies to talk to quarterbacks because there is no cut-off switch. Communication between sideline and the field is cut off with 15 seconds remaining on the play clock.

Normal protocol during times when the radio system malfunctions is for it to be shut off for both teams so neither has an advantage.

The source said the Cowboys did not file a complaint but that league officials noticed the infraction while watching the game on television. The Giants face a possible fine for the incident.

The Giants did not comment on the matter.