According to various sources, a NFL investigation has found that the New England Patriots used under-inflated footballs in the AFC championship game. Apparently the NFL found 11 balls were not properly inflated.

According to NFL rules, footballs are required to be inflated between 12.5 and 13.5 pounds per square inch and weigh between 14 and 15 ounces.

Here's interception that launched the investigation. The Colts noticed that the ball felt under inflated and notified the league.