The National Football League has determined that 11 of the New England Patriots' 12 allotted balls used in Sunday's AFC title game were under-inflated by two pounds each, ESPN reported.
ESPN cites league sources, though Greg Aiello, the NFL's senior vice president of communications, told the media outlet the league was not ready to comment.
Earlier, an NFL spokesperson said the investigation into the matter should be completed within a few days.
Patriots quarterback Tom Brady has dismissed claims as "ridiculous."
The Patriots beat the Indianapolis Colts 45-7 to advance to the February 1 Super Bowl, where they will meet the defending champion Seattle Seahawks.
A softer ball could be easier to grip and catch and therefore potentially might have helped the Patriots.
Each team uses its own balls when its offense is on the field.
The NFL requires balls to be inflated between 12.5 and 13.5 pounds per square inch and weigh between 14 and 15 ounces.
The NFL investigation still needs to find out how the 11 balls came to be under-inflated, says ESPN.
The league must also decide whether to impose a penalty on the Patriots and, if so, what it should be.