Two prominent concussion researchers -- including a senior adviser to the NFL -- served as paid consultants to law firms suing the league for allegedly concealing the link between football and brain damage, according to interviews and documents obtained by "Outside the Lines" and "Frontline."

Dr. Robert Cantu, an unpaid senior adviser to the NFL's Head, Neck and Spine committee and a Boston University researcher, delivered a two-hour medical overview to dozens of attorneys attending a 2012 strategy session at a Philadelphia law firm leading the suit against the NFL, Cantu confirmed.

Cantu's colleague at Boston University, Chris Nowinski, also has served as a consultant on the lawsuit, receiving thousands of dollars for advising lawyers on concussion-related matters and potentially testifying on behalf of players suing the league, documents and interviews show.

The disclosures underscore the complex relationships of the Boston University research team, which received a $1 million gift from the NFL in 2010 to explore the connection between football and concussions but has since come under criticism from league-affiliated doctors for stating definitively that the sport causes brain damage. Over the past several months, the NFL has distanced itself from Boston University, which continues to press its case about the devastating effects of pro football, a central issue in the lawsuit.

Oral arguments begin Tuesday in U.S. District Court in Philadelphia on the NFL's motion to dismiss the lawsuit, which involves more than 4,000 former players who allege the league concealed the effects of football-related head trauma.

Brain researchers such as Cantu and Nowinski will be key players if the lawsuit moves forward, and both sides are engaged in a fierce competition to recruit them. The outcome of the case will likely turn on the NFL's own controversial concussion research, and the ability to sign up expert witnesses -- or get them disqualified -- has become a major subplot in the ongoing drama.

Cantu, 74, has long been one of the nation's leading experts on sports-related concussions; in the mid-1980s, he developed the first guidelines for when an athlete should return to play. He also edited a series of now-discredited scientific papers published by the NFL in a leading medical journal. Nowinski, 34, became an activist and researcher after Cantu, his doctor, diagnosed him with concussion symptoms stemming from his career as a professional wrestler. His story was featured in last year's documentary film, "Head Games."

Cantu notified the NFL about his appearance before the lawyers opposing the league sometime after the February 2012 presentation at the offices of Anapol Schwartz, a Philadelphia firm. Cantu, in an interview, described the presentation as "an educational talk" in which he shared his views on concussions and chronic traumatic encephalopathy, a neurodegenerative disease that researchers have linked to football.

"It was an informational session, just like I get paid to give a talk someplace else," he said.

Cantu was not retained as an expert witness by the lawyers after attorneys who attended the session pointed out that he worked with the NFL; some were angry that the potential conflict had not been addressed beforehand and became concerned that his dual roles could harm the case.

In fact, "Outside the Lines" and "Frontline" have learned that within three months of Cantu's appearance, the NFL sent a letter to plaintiffs' attorneys expressing its own concerns. The letter effectively told the lawyers to stay away from Cantu, noting his role as a senior adviser to the concussion committee. The NFL warned that Cantu's relationship with the plaintiffs could result in the lawyers receiving confidential information related to the litigation. The letter also stated that Cantu had signed a document assuring the NFL he had no conflicts of interest that would prevent him from fulfilling his responsibilities with the committee.

It is unclear how much Cantu was paid for his presentation; his fee schedule as an expert witness indicates he receives $800 an hour for legal services, $5,000 for depositions and $8,000 a day for testifying during a trial, plus travel and hotel expenses.

"I was giving the science as I knew it," Cantu said. He added that had the NFL retained him as an expert witness, he would have been prevented from advising lawyers for the players. But he said he did not see his role as adviser to the league's concussion committee as a conflict.

Cantu said he often works as an expert witness and "if [the NFL] wanted to put me on their payroll, to defend their case, then I'm not gonna say boo about those issues [to the plaintiffs]."

Dr. Robert Cantu during a U.S. House of Representatives Judiciary hearing on head injuries among NFL players. Scott J. Ferrell/Congressional Quarterly/Getty Images

In a statement, an NFL spokesman praised Cantu as "an important adviser" and said the league resolved the issue: "After discussing the matter with Dr. Cantu, we took appropriate steps to permit us to continue our work with him. Dr. Cantu's work with our medical committee is ongoing, and we are grateful for his service."

According to documents obtained by "Outside the Lines" and "Frontline," Nowinski, before his affiliation with Boston University, was retained for $5,000 by a group of lawyers to "perform litigation services relating to potential and/or actual claims that may be asserted by one or more of our clients against the National Football League and others." Those services included consulting and potentially testifying against the NFL. The relationship was ended because of differences over the work to be performed, according to a source.

Nowinski, when asked about information indicating that he was currently doing consulting work for lawyers suing the NFL, said: "Where did you hear that?" After a pause he replied: "I have no comment."

In a follow-up email, Nowinski wrote that all of his consulting agreements are vetted by the board of directors of the Sports Legacy Institute, a nonprofit research organization founded by Nowinski and Cantu that works in tandem with Boston University.

"We were not aware of Mr. Nowinski's relationship with any plaintiff or plaintiff's attorney," the NFL spokesman said. "Had we known, it would not have affected our decision to provide a $1 million grant to Boston University to support its research. Our goal is to support a wide range of independent scientific and medical research and to let the outcome of that research guide our decisions."

Researchers often are asked to appear as expert witnesses in legal proceedings related to their fields. The NFL suit, with the potential for billions of dollars in damages, has created a large demand for researchers with expertise in the science of concussions.

But some researchers said they have turned down such requests despite the potentially lucrative payoff out of concern the perceived conflict could compromise their research. Kevin Guskiewicz, who runs the Matthew Gfeller Sport-Related Traumatic Brain Injury Research Center at the University of North Carolina, said he has declined numerous requests to serve as a consultant on the concussion suit to avoid such potential conflicts. Guskiewicz serves on concussion committees for both the NFL and the players' association.

Warren Dunn, an assistant professor of orthopedic surgery who has written on medical ethics and sports medicine, said Cantu's work with the NFL committee and lawyers representing the players "seems awkward to me," giving the impression that Cantu was straddling the line between an "industry and the people who claim they were harmed by that industry."

While Cantu said he is not currently working for either side in the NFL litigation, he acknowledged he has been retained as an expert in a similar lawsuit filed against the NCAA. Cantu declined to say which side hired him, though a lawyer representing the ex-college players said they had retained Cantu. Cantu said he entertained offers from both the plaintiffs and the defendants.

The NFL's concussion research is at the heart of the lawsuit against the league. The NFL's Mild Traumatic Brain Injury committee for years denied a connection between football and brain damage. Since 2009, after disbanding the committee, the NFL has poured more than $100 million into concussion research, partnering with institutions as varied as the National Institutes of Health, the U.S. Army and General Electric.

In part because of its NFL-funded research, the Boston University group has gained international recognition and millions of dollars in funding for its work on CTE. The group has identified the disease in 33 deceased NFL players. Along with the U.S. Department of Veterans Affairs, BU maintains a brain bank to which at least 150 athletes, including 40 active or retired NFL players, have pledged their brains for scientific research.

Chris Nowinski says that all of his consulting agreements are vetted by the board of directors of the Sports Legacy Institute, a nonprofit research organization founded by Nowinski that works in tandem with Boston University. AP Photo/Steven Senne

But Boston University has faced growing criticism from other neuroscientists -- including some not affiliated with the NFL -- who believe the group has created unwarranted hysteria over the risks of playing football by hyping their findings. Some researchers question whether other factors contribute to CTE and note that the prevalence of the disease has never been established.

In an interview with "Outside the Lines" and "Frontline," Dr. Mitchel S. Berger, a member of the NFL Head, Neck and Spine Committee, described BU's claims about the link between football and brain damage as "irresponsible" and "biased" and suggested the findings were shaped by self-interest.

"The BU Group, their whole existence -- their funding -- relies on perpetuating that it's a fact if you play football you're going to have some form of cognitive impairment," said Berger, who chairs the Department of Neurological Surgery at University of California at San Francisco. "So it's very, very difficult to accept it because it is so biased."

Cantu, when told of Berger's comments, said: "Mitch Berger, with all due respect, is full of s---. No, not with respect."

Cantu suggested that Berger's criticisms stemmed from professional jealousy over publicity received by the Boston University group and Dr. Ann McKee, a neuropathologist also affiliated with Veterans Affairs who made the diagnoses of CTE in former players. "This is a neuropathological diagnosis that's black and white, and one confirmed by anyone who has looked at the tissue," Cantu said. "It's not something with bias. It's not like if this brain doesn't have it, we'll duck it or stick it in a bucket."

Nowinski described Berger's comment as "bizarre. I mean, the facts are the facts. We have 33 former NFL players [with CTE], just from our group." Referring to Berger's affiliation with the NFL, Nowinski also noted that the criticism was coming from "somebody connected with the group that profits from the sport ... ."

The NFL acknowledged the connection between football and long-term cognitive issues in December 2009 but lately has retreated from that assertion. Asked if he thought the league's shifting public statements were related to the lawsuit, Nowinski said: "It's tough to say. I wouldn't imagine that the NFL doctors would have their opinions affected by the lawsuit. I'm sure it's on the minds of a lot of people."

The NFL's $1 million gift to Boston University in 2010 came in the wake of a congressional hearing in which politicians criticized Commissioner Roger Goodell for the league's efforts to deny a link between football and brain damage. In addition to the donation, the NFL pledged to direct the brains of deceased former players to the Boston University group. Goodell said he hoped the partnership would "lead to a better understanding of these effects and also to developing ways to help detect, prevent and treat these injuries."

But the league appears to be moving in another direction: Berger said the NFL is now hoping to set up a brain bank with the National Institutes of Health, which received a $30 million gift from the league in September.

Last May, when former San Diego Chargers linebacker Junior Seau committed suicide in San Diego, representatives of the NFL directed the brain to the NIH. Although Seau was later diagnosed with CTE, Boston University was not involved in studying his brain.