Three months after House Democrats issued a report concluding that the NFL improperly sought to influence a major government study on football and brain disease, their Republican counterparts called for an independent review into the controversy.

In a 21-page letter to the inspector general of the Department of Health and Human Services, Republican leaders from the Energy and Commerce Committee requested the review to determine whether proper procedures were followed by officials with the National Institutes of Health (NIH) and its fundraising arm.

The letter suggests that NIH officials engaged representatives from the NFL throughout the process and tried to resolve the conflict by offering a compromise solution -- "reinforcing the perception that there was nothing improper" about the NFL's actions.

"The questions and concerns raised by these events are vital to the integrity of research and the grant award decision-making process," concluded the letter, written by Reps. Fred Upton of Michigan, Tim Murphy and Joseph Pitts of Pennsylvania, and Michael Burgess of Texas.

In May, Democratic leaders from the committee issued a 91-page report that described a behind-the-scenes campaign by top NFL health officials to strip a $16 million grant from a prominent Boston University researcher who has been critical of the league. The Republican letter provided little new information about the NFL's campaign, but it suggested that the league may have been convinced that its input was welcomed by senior NIH officials who agreed to discuss the matter, in possible violation of NIH regulations.

The Democratic report stemmed from interviews with officials at the NIH and the Foundation for the NIH (FNIH) as well as the NFL's top health and safety official. It also cited emails between the parties. The report singled out Dr. Richard Ellenbogen, the co-chair of the NFL's Head, Neck and Spine Committee. Ellenbogen was a co-investigator on a competing grant proposal, which the report said NFL officials tried to convince the NIH to fund. Ellenbogen was "a primary example of the conflicts of interest between his role as a researcher and his role as an NFL adviser," according to the report.

Ellenbogen, however, was not interviewed by the Democrats, and he denied to the Republicans that he ever had sought to influence the NIH process, as he had previously to Outside the Lines and other media outlets.

Ellenbogen, the chair of the Department of Neurosurgery at the University of Washington, is the subject of an internal investigation at UW over the findings by the Democrats.

The Republican letter said the review was necessary because "this controversy has tarnished the reputations" of some leading brain trauma experts.

"We stand by our findings that the NFL improperly attempted to influence NIH on its brain injury research, and that FNIH did not adequately fulfill its role of serving as an intermediary between NIH and the NFL," an Energy and Commerce Democratic Committee spokesperson wrote to Outside the Lines.

The controversy stems from a $30 million donation the NFL made to the NIH in 2012. At the time, the league described the grant as "unrestricted," and NFL commissioner Roger Goodell insisted that the league would let NIH make the decisions on how best to spend the money. The first $12 million of that funding was allocated in 2013 to two groups to study the defining characteristics of chronic traumatic encephalopathy (CTE), the brain disease that has been found in dozens of deceased NFL players and that experts have linked to repetitive head trauma. However, Outside the Lines reported in December that the NFL had backed out of using most of the remaining funds on a $16 million study that had been awarded to Robert Stern, a prominent Boston University researcher.

Soon after, the Democratic committee launched its investigation.

The league has said that its officials expressed concerns about Stern but that they did so appropriately. And the Republicans' letter suggests that if the NFL's actions were inappropriate, the NIH should have addressed that with the league rather than "encouraging ongoing discussion."

The NFL issued a statement Thursday afternoon repeating prior comments that it had never "wavered in its commitment to advance the science and understanding of concussions" and that it looked forward to cooperating in a review.

Ellenbogen and three other members of the NFL's Head, Neck and Spine Committee issued their own statement, saying they "welcome a fair and balanced review" and "look forward to engaging fully in the process, a courtesy we were not provided by the committee's minority staff."

The Republican letter comes one day after the NFL announced a $100 million initiative designed to focus on concussion research and technologies to lessen head trauma.