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The National Institutes of Health will reportedly let its partnership with the NFL expire in August after the NFL previously pledged $30 million to help research the connection between brain disease and football.

According to ESPN's Mark Fainaru-Wada and Steve Fainaru, the NIH decided not to renew its agreement with the NFL "following a bitter dispute in 2015 in which the NFL backed out of a major study that had been awarded to a researcher who had been critical of the league."

"The NFL's agreement with [the funding arm of the NIH] ends August 31, 2017, and there are no current research plans for the funds remaining from the original $30 million NFL commitment," the NIH said in a statement, per ESPN.

Confirmation of the severed relationship between the NFL and NIH comes two days after the Washington Post's Mark Maske reported Democratic members of the House Committee on Energy and Commerce submitted a letter to the NFL asking if it planned to fulfill the terms of its donation.

According to Maske, the NFL has still not contributed $18 million of the initial amount pledged in 2012.

"We are currently engaged in constructive discussions with the FINH regarding potential new research projects and the remaining funds of our $30 million commitment," the NFL said Wednesday in a statement provided to Bleacher Report by CNN's Jill Martin. "In September 2016, the NFL pledged $100 million in support for independent medical research and engineering advancements in neuroscience-related topics. This is in addition to the $100 million that the NFL and its partners are already spending on medical and neuroscience research."

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The relationship between the NFL and NIH has reportedly been contentious for years.

According to a May 2016 investigation by the New York Times' John Branch, the House Committee on Energy and Commerce conducted a study that detailed the NFL's attempt to influence where its donation was funneled.

"Our investigation has shown that while the NFL had been publicly proclaiming its role as funder and accelerator of important research, it was privately attempting to influence that research," the committee wrote. "The NFL attempted to use its 'unrestricted gift' as leverage to steer funding away from one of its critics."

On Tuesday, the medical journal JAMA (via CNN's Daniella Emanuel) published the results of a study that showed chronic traumatic encephalopathy (CTE) had been found in the brains of 110 of 111 deceased former NFL players.