(Reuters) - 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 has 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 Patriots built a 17-7 lead in the first half on Sunday's game and outscored the Colts 28-0 in the second half.

The growing storm over the team's 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, game balls must be inflated to between 12.5 and 13.5 pounds per square inch and no alteration 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.

Patriots owner Robert Kraft said investigators spent three days at the team's facility during which they were given access to every full- and part-time employee they requested to speak with and every communication device that they wanted to search.

"Competitive balance and the integrity of the game are the foundation of what makes our league so special and I have the utmost respect for those principles," Kraft said in a statement.

"Our organization will continue to cooperate throughout the league's investigation."

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 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.