ESPN the Magazine senior writer Peter Keating examines possible explanations for why over 100 diagnosed concussions weren't included in head-injury studies conducted by the NFL. (3:02)

Four members of Congress have asked the NFL for documents and information about why it tried to intervene in the selection of a Boston University researcher to lead a major study on football and brain disease, according to a letter obtained by Outside the Lines.

The letter, which was sent Wednesday to commissioner Roger Goodell, includes new information showing how the NFL engaged in a monthslong campaign to derail the selection of Dr. Robert Stern, a longtime critic, and replace him with researchers affiliated with the league.

The $16 million study was to be funded out of a $30 million "unrestricted gift" the NFL gave the National Institutes of Health in 2012. The league ultimately withheld funding after the NIH rejected the NFL's claims that Stern was biased and his selection marred by a conflict of interest.

"Efforts by outside entities to ... exercise influence over the selection of NIH research applicants are troubling, and we are committed to a full understanding of the sequence of events that led to this dispute," the legislators wrote. The letter was signed by Reps. Frank Pallone Jr., D-N.J.; Gene Green, D-Texas; Diana DeGette, D-Colo.; and Jan Schakowsky, D-Ill. of the House Energy and Commerce Committee.

The investigation was triggered by an Outside the Lines report in December revealing how the NFL had reversed a commitment to fund the study, which aims to find methods for diagnosing chronic traumatic encephalopathy, or CTE, in living patients.

Both the NFL and the Foundation for the NIH -- a nonprofit organization that administers the league's $30 million grant -- denied that the NFL had intervened.

Additional stories by Outside the Lines revealed that members of the NFL's Head, Neck and Spine Committee were applicants for the grant that went to Stern. They included the principal investigator, Kevin Guskiewicz, a prominent University of North Carolina concussion researcher who chairs the NFL's Subcommittee on Safety Equipment and Playing Rules. If the Guskiewicz proposal had been chosen, millions of dollars would have been directed to institutions that employ the league's advisers.

Outside the Lines also reported that senior NFL health and safety officials contacted the NIH in an effort to get the decision reversed. But the emails referenced in Wednesday's letter reveal an effort to pressure the NIH that was broader and more sustained than was previously reported.

In June, Dr. Elliot Pellman, an NFL adviser who once headed the league's concussion research, asked the FNIH to "communicate our concerns and slow down the process until we have a chance to speak to figure this out." In August, the league's chief medical officer, Dr. Betsy Nabel, recommended distributing the award among other researchers to "dilute the voice of a more marginal group," according to the letter.

In October, Dr. Maria Freire, president and executive director of the FNIH, warned Jeff Miller, the NFL's senior vice president for health and safety, that refusing to fund the CTE study would take money from other important research led by the National Institute for Neurological Disorders and Stroke (NINDS), the NIH institute overseeing the study, according to an email provided to the committee.

Instead of relying on the NFL funding, "supporting the CTE study with taxpayer dollars means that NINDS will be unable to fund other meritorious research for several years," Freire wrote. "As I mentioned on the phone to you, we think it is important for NFL to contribute to this study."

In their letter to Goodell, the legislators wrote: "Despite this information, the NFL did not commit to fully funding the CTE study, according to the documents and briefings we have received."

Green, the Texas congressman and one of the letter's signatories, told Outside the Lines last week: "The NFL usually gets their way. But that's not how you get the real valuable information. You don't game the study, particularly through NIH."

"The NFL played, I think, an unscrupulous role," Schakowsky said an interview Thursday. "It's like, 'Here's an unrestricted grant and, oh, by the way, we don't like the researcher.' That's very troubling."

Dr. Walter Koroshetz, director of the NINDS, previously told Outside the Lines that the institute's advisory council had concluded that the NFL's allegations against Stern were baseless. With no indication that the NFL would make the funding available, the NIH chose to use taxpayer money to keep the study alive.

The new disclosures come at a critical time for the league. Last week, during a "concussion roundtable" organized by the House's Energy and Commerce Committee, Miller responded to a question from Schakowsky and acknowledged a link between football and CTE, the first time an NFL official has conceded the sport's connection to a devastating disease that has afflicted dozens of former NFL players.

An NFL spokesman told Outside the Lines on Thursday that "Goodell will respond to the committee's letter." Earlier Thursday, the NFL issued a lengthy response to a New York Times article about how the league handled concussion data, saying that "We have committed tens of millions of dollars to fund independent research" and that the league has an "ongoing commitment on the issue of player health and safety -- notably, to the support of research, including that of our most vocal critics."

For nearly two decades, the NFL denied such a connection in its own scientific papers and attacked independent researchers who asserted otherwise. The Times reported Thursday that the NFL omitted more than 100 diagnosed concussions in data that formed the basis for some of the NFL's previous research.

More recently, the league and its corporate partners have spent over $100 million to fund dozens of research studies, including the $30 million gift to the NIH.

That donation came with no strings attached, the league announced, but it since has become clear that the NFL has gone to unprecedented lengths to wield its influence over the CTE study, which is believed to be the most exhaustive examination to date about the relationship between football and neurodegenerative disease.

After Stern's selection, the NFL encouraged the NIH to broaden the award to include two other groups of researchers. Nabel, a cardiologist who is president of Brigham and Women's Hospital in Boston, wrote that "a collaboration of the three groups would be ideal."

According to the letter, the NINDS advisory council rejected that proposal because "none of the other grant applications had adequate scores to justify funding an additional group of researchers."

In the letter to Goodell, the committee members asked the NFL to explain why the league thought it was appropriate to weigh in on the grant review and why the NFL withheld funding for the study. It also asked for information about the Head, Neck and Spine Committee and Pellman, the former head of the league's disbanded research arm.

Said Pallone on Thursday: "It is critically important that the research conducted at NIH is not compromised by outside groups or their interests. In order to ensure that the integrity of the scientific process is protected, we need answers from the NFL."