A group of House Democrats are questioning whether the National Football League tried to influence the selection of scientists for a $16 million National Institutes of Health-funded study on brain injuries, adding a new layer of scrutiny to the professional sports league’s sometimes contentious relationship with Congress.

In a letter to NIH Director Francis Collins sent Thursday, top Democrats on the Energy and Commerce Committee pressed the agency for more information about a $30 million grant that the NFL gave to NIH for scientific studies in 2012.

Ranking member Frank Pallone Jr. of New Jersey, Gene Green of Texas, Diana DeGette of Colorado and Jan Schakowsky of Illinois said they were concerned over a recent ESPN report that the league backed out of funding a seven-year study examining chronic traumatic encephalopathy, or CTE, a neurodegenerative disease associated with head trauma, after  NIH awarded the money to a Boston University researcher who has criticized the NFL.

The questions come after Energy and Commerce announced last month it will conduct a broad review of concussions, including those related to professional and collegiate sports.

“The NIH’s independent peer review process forms the cornerstone of the NIH research mission and ensures that applications submitted to the NIH are evaluated by scientific experts in a manner free of inappropriate influences or bias,” the lawmakers wrote. “We are concerned about the potential implications of outside entities attempting to exercise ‘veto power’ or other influence over the selection of NIH research applicants.”

ESPN reported last month that even though the NFL is not supposed to have any control over how the NIH grant money is spent, the league objected to a study on diagnosing CTE because they questioned the researcher’s objectivity and backed out of funding the project.

The NFL disputed that it pulled money from the study, which raises the question of whether the league fully committed to funding the project in the first place. The league maintains that it did not have any veto power over the grant. Lawmakers are seeking to reconcile the conflicting reports.

“We both value and understand the importance of independent research,” the NFL said in a statement. “We are committed to helping accelerate research to help find answers.”

The BU study is moving forward and will be funded by NIH. Pallone said Friday he remains concerned and wants to ensure outside groups are not able to compromise the independence of the nation’s biomedical research agency.

“This is a very important study, and I don’t want it to be tainted in any way by some veto policy of the NFL,” Pallone told CQ Roll Call. “The larger question is in the future: I don’t think it’s appropriate to have donors dictate who the researchers are.”

Democrats also sent a letter to the non-profit Foundation for the National Institutes of Health, which helped arrange the donation. The lawmakers are seeking information about the agreement between NFL and NIH, including any documents and emails between employees regarding the conditions of the $30 million grant, by Feb. 1. The NIH indicated it has received the letter and will respond to lawmakers promptly.

The incident is the latest in a string of public relations headaches for the NFL. The league has come under fire in Congress for its handling of domestic violence, its tax-exempt status and, most recently, connection to daily fantasy sports web sites and gambling.

Part of the NFL’s Washington strategy has involved currying goodwill among lawmakers by going on the offensive on some issues and making small concessions on others. The league – which spent $920,000 on lobbying in the first three quarters of 2015 and $1.2 million in 2014 – voluntarily gave up its tax-exempt status and hosted a safe-tackling demonstration for youth football in the Rayburn House Office Building last year, for example.

But that may not be enough for some lawmakers.

“I’m from Texas. I love football,” Green said Friday. “I’m glad the NFL is stepping up the concussion protocols that they have, but I watch the games too and it may not be enough. We need more science.”

Kate Ackley contributed to this report.