The NFL said on Friday that its ongoing investigation into the New England Patriots has shown that the team used balls that were under inflated during a victory that clinched a Super Bowl berth.
But the league said it had not made any judgments and will not do so until it concludes the investigation and considered all of the relevant evidence.
After nearly 40 interviews, the NFL said evidence supports the conclusion that illegal balls were used by the Patriots in the first half but were properly inflated for the second half of their 45-7 win over the Indianapolis Colts on Sunday.
"The goals of the investigation will be to determine the explanation for why footballs used in the game were not in compliance with the playing rules and specifically whether any noncompliance was the result of deliberate action," the NFL said in a statement.
The growing storm over the Patriots' use of illegal balls, which is being referred to as 'deflate-gate,' has dominated the buildup to the Feb. 1 Super Bowl versus the Seattle Seahawks.
Under NFL rules, no alteration of game balls, which must be inflated to between 12.5 and 13.5 pounds per square inch, is allowed once they are approved.
Patriots quarterback Tom Brady threw three touchdown passes during Sunday's game in wet conditions where a firm grip on a slippery ball could have helped.
Both Brady and Patriots head coach Bill Belichick have professed innocence regarding the deflated ball issue and said they knew nothing about it until Monday.
The NFL said it still needed to obtain more information, including video and other electronic information and physical evidence, before wrapping up its investigation.
The league also said it had retained an investigatory firm with sophisticated forensic expertise to assist in reviewing electronic and video information.
"The playing rules are intended to protect the fairness and integrity of our games. We take seriously claims that those rules have been violated and will fully investigate this matter without compromise or delay," the NFL said.
"The investigation is ongoing, will be thorough and objective, and is being pursued expeditiously."
The investigation is being led jointly by NFL Executive Vice President Jeff Pash and lawyer Ted Wells, who oversaw the league's probe into the Miami Dolphins' bullying scandal.
The NFL also said the Patriots have pledged their full cooperation and have made their personnel and other information available upon request.
"Our investigation will seek information from any and all relevant sources and we expect full cooperation from other clubs as well," the league said. "As we develop more information and are in a position to reach conclusions, we will share them publicly."