An NFL investigation reportedly has determined that 11 of the 12 game footballs used by the New England Patriots during Sunday's AFC Championship game were under-inflated by two pounds per square inch below the required minimum.

In an NFL letter about the investigation that was shared with the Boston Globe, the Patriots were informed that initial findings indicated their game balls did not meet requirements, the newspaper reports.

ESPN reported late Tuesday that the league had made the determination earlier in the day and described NFL officials as "disappointed", "angry", and "distraught." However, it is not known how the balls became under-inflated. 

Under NFL rules, each team is provided with 12 balls to use on offense during the game. The balls are required to made available for inspection and approval by the game referee two hours and fifteen minutes before kickoff. The balls are required to be inflated between 12.5 and 13.5 pounds per square inch and weigh between 14 and 15 ounces. Teams are not allowed to alter the balls in any way once they are approved by the officiating crew. 

The Boston Globe reported late Tuesday that the officiating crew for Sunday's game, led by referee Walt Anderson, discovered at halftime that the game balls were under-inflated after testing them with two different gauges. 

Earlier Tuesday NFL senior executive vice president for football operations Troy Vincent told ProFootballTalk that the league expected to complete its investigation in "the next two or three days." The league did not comment publicly when reached by Fox News Wednesday morning. 

The Patriots defeated the Colts 45-7 to advance to their sixth Super Bowl since 2001. They will play the Seattle Seahawks in the Super Bowl on Feb. 1 in Glendale, Ariz.