Tagliabue confirmed Wednesday he had been treated by Pellman, but not until 1997, three years after he had appointed Pellman to lead the concussion committee. "No personal medical care had anything to do with Dr. Pellman's appointment to the committee in 1994," the former commissioner said in a statement released by NFL spokesman Greg Aiello. Aiello said Tagliabue saw Pellman "on occasion" as a patient for nine years until Tagliabue retired in 2006.

Pellman's relationship with Tagliabue is certain to be explored thoroughly if the lawsuit filed by more than 4,800 retired players against the NFL moves forward. The league has distanced itself from the MTBI committee, asserting that its work was independent. The league also says its Head, Neck and Spine Committee, which replaced the MTBI group, operates independently of the league office. Last month, a judge ordered the two sides in the lawsuit to mediation to seek a settlement.

"This is something that should scare the hell out of the NFL as part of the concussion litigation," Warren Zola, a sports law expert and assistant dean at Boston College, said when told of Pellman's doctor-patient relationship with Tagliabue.

As a veteran team doctor with experience treating concussions, Pellman might have been qualified to lead the committee, but his relationship with Tagliabue could undermine his credibility, Zola said.

"As a matter of law, I'm not sure it would be all that damning," Zola said. "But if the NFL were to find themselves in front of a jury, the jury would likely interpret this as evidence of negligence. It's another rationale for the NFL to try to settle."

In 2005, the New York Times revealed that Pellman embellished his credentials and failed to disclose that he attended medical school in Guadalajara, Mexico. Pellman acknowledged the mistakes, which he said were unintentional and primarily made by people he worked with.

Pellman, who joined the Jets in 1988, wrote in October 2003 that Tagliabue invited him to lead the NFL's research after Pellman treated Al Toon, a Jets receiver forced into retirement in 1992 -- the first in a series of prominent head injuries that led to players retiring prematurely.

"The commissioner and I realized that we had many more questions than answers," Pellman wrote in the medical journal "Neurosurgery." "I was asked to mount an effort to answer these questions."

Tagliabue retired in 2006. The following year, Pellman stepped down as chairman of the MTBI committee, although he was retained as a committee member. Pellman's doctor-patient relationship with Tagliabue was known among some committee members.

Tagliabue, in his statement, said that he had "never heard of Elliot Pellman until his service as the Jets' team physician came to my attention. His appointment to the committee was based on his experience in sports medicine, his work with the Jets that included Toon's concussion-related retirement and Dennis Byrd's spinal cord injury, and recommendations from Jets ownership and management."

Pellman declined repeated interview requests for this story, but in a brief telephone exchange said information about whether he is or was Tagliabue's doctor "is between myself and Paul Tagliabue. I can't talk about that."

In 2003, the NFL -- with Pellman still in charge of the committee -- published the first of 16 studies, many of which suggested concussions were not a significant problem in the NFL. Tagliabue had previously expressed skepticism about the seriousness of the league's concussion problem.

Pellman was the lead author in nine of the studies. The league concluded repeatedly that no NFL player had suffered brain damage. In 2005, Pellman and two colleagues on the MTBI committee tried unsuccessfully to force the retraction of a peer-reviewed paper asserting that football gave Pittsburgh Steelers Hall of Fame center Mike Webster brain damage. The demand was seen as highly unusual in a scientific journal because such actions are normally reserved for transgressions such as fraud or plagiarism.

The NFL has since reversed course, implementing rule changes that in some cases directly contradict the league's earlier findings. Concussed players are no longer allowed to return in the same game, for example. In 2005, a Pellman-led NFL study concluded "many NFL players can be safely allowed to return to play on the day of injury after sustaining an MTBI."

While league spokesman Greg Aiello told "Outside the Lines" and "Frontline" that Pellman's current role is as an adviser in an administrative capacity, Pellman has been identified as "NFL Medical Director" in documents and continues to attend meetings of the Head, Neck and Spine Committee.

In December 2011, Pellman was described as the "NFL Medical Director" in an email from the co-chairman of the current concussion committee to its members. The email, obtained by "Outside the Lines" and "Frontline," was sent by Dr. Richard Ellenbogen, a Seattle neurosurgeon who identified Pellman as a co-sponsor of a committee meeting last year at the NFL's New York headquarters along with Jeff Pash, the league's executive vice president. Aiello said last week the discrepancy between Pellman's titles boils down to "semantics."

Pellman "does not establish policy," Aiello said. "In conjunction with committee chairmen, he assists in the administration of the committees. This does not include the Head, Neck, and Spine Committee, which is administered by the co-chairs. … He does follow up with team doctors for information on significant injuries that he reports to our office."

Former Jets team doctor Elliot Pellman, center, served as former NFL commissioner Paul Tagliabue's personal physician for nearly a decade -- even as Pellman led the Mild Traumatic Brain Injury Committee, which shaped the NFL's concussion policies. AP Photo/Ed Betz

Aiello also pointed out that Pellman is an adviser to Major League Baseball and is the medical director of the New York Islanders. The website Sports on Earth first reported in May that Pellman still works for the NFL.

Some of Pellman's colleagues seem uncertain why he retains his position.

"It's a question that many of us have asked," Kevin Guskiewicz, a prominent University of North Carolina neuroscientist who serves on the Head, Neck and Spine Committee, told "Outside the Lines" and "Frontline." "I mean, I really don't know."

When asked why the league still employs Pellman, Aiello accused an ESPN reporter of "being on a witch hunt."

While Pellman's continued employment in the league might be relevant to the players' lawsuit, so, too, could the questions some players have about Pellman's overlapping roles as head of the NFL's concussion committee and Jets team doctor. Former players said that in the heat of games, Pellman had players recite a three-word code -- "Red Brick Broadway" -- to determine if they were able to play after a blow to the head. The code became "kind of a running joke in the locker room because the three words were always the same," said Kevin Mawae, who played center for the Jets from 1998-2005. "He would leave you and come back before the next series and you'd go, 'Red Brick Broadway. I'm ready to go.' "

Mawae said it was "pretty common" for Jets players to return to play after suffering a head injury during Pellman's tenure "unless the guy was just out and carted off the field or something like that."

Kyle Brady, a former Jets tight end who has joined the lawsuit, said: "I don't think the Jets handled it any differently than probably the way the majority of teams in the league were handling it at that time."

Brady described how Pellman allowed him to return after Brady was knocked unconscious while reaching for a pass from Vinny Testaverde during a 1999 playoff game against the Jacksonville Jaguars.

Brady chuckled as he recalled being treated on the sideline while head coach Bill Parcells was "kind of hollering at the guys, saying, 'Is he gonna be all right? When's he gonna get back in there?' You know, in a classic Parcells kind of way. I'm not sure if it was a question. It might have been a command."

Brady said Pellman allowed him to return for the next offensive series, still in a daze. Brady acknowledged that he was as eager as Parcells to get back in. "At that point, you're kind of like a slobbering dog," he said.

Former player Kevin Mawae said it was "pretty common" for New York Jets players to return to play after suffering a head injury during Dr. Pellman's tenure, "unless the guy was just out and carted off the field or something like that." AP Photo/Scott Boehm

By then, Pellman and the MTBI committee were five years into their research -- and the earliest studies by other researchers suggesting a possible link between football and long-term brain damage were just being completed.

In 2000, Pellman was named as a potential expert witness to defend a Chicago Bears doctor, John Munsell, who was being sued by running back Merril Hoge over the treatment of Hoge's career-ending head injuries, court documents show. The case was the first concussion suit to involve the NFL and foreshadowed the massive litigation now unfolding in federal court in Philadelphia.

Hoge, during a 1994 preseason game at Kansas City, sustained a concussion so severe he told trainers he believed he was in Tampa Bay because he could "hear the ocean." Within a few weeks of being cleared to play, Hoge, now an ESPN analyst, suffered another concussion that ended his career. Pellman was prepared to testify that Munsell acted within the NFL's "standard of care" when he allowed Hoge to practice without a follow-up exam, according to a letter from Munsell's lawyer submitted to the court. Pellman was later disqualified on a technicality and did not testify. Hoge initially won a $1.55 million judgment but settled after the ruling was set aside.

In 2003, Pellman faced pointed questions from the media about his treatment of wide receiver Wayne Chrebet, who was allowed to re-enter a close game against the New York Giants after being knocked out. Chrebet, who retired in 2005, was never the same caliber of player.

But Mawae and Brady said such occurrences were not uncommon. On one occasion, said Mawae, Pellman cleared him to play after Mawae, like Chrebet, took a knee in the head and was knocked unconscious. Mawae went back in the following series, but he later recalled almost nothing about the game and had symptoms for days afterward.

"In hindsight, if I had to do it again and knowing what we know now about concussions and brain trauma and stuff like that, the smart thing would have been to probably stay on the sideline," Mawae said.

Mawae said he liked and respected Pellman "as a man," adding that he previously used him as his family doctor.

"Pellman's just a small part in this," said Mawae, who was a longtime member of the NFLPA's executive committee and served as its president from 2008 through 2012. "The bigger part of this is it's a multibillion-dollar industry that cannot afford to have something such as [concussions causing brain damage] shut the league down. And so anything you can do to suppress that information to make everybody think it's gonna be OK, then that's what we'll do."

Mawae said he has decided not to join the lawsuit because "from a very early age I always knew there was an inherent risk of injury.

"And even today, if I were to step foot on the field, I would be going on the field knowing that I can be maimed, hurt or killed playing this game. I sit in a beautiful home and my kids' education is paid for. My children's children will be OK because of the risk that I was willing to take at the highest level. It was an educated decision on my part knowing that I could be a vegetable but my family would be taken care of."

Brady, who completed his law degree in May and has joined the suit, said the reasoning behind the lawsuit it simple: "It's about justice. It's about doing what's right at the end of the day, and accountability."

Reporter Mark Fainaru-Wada and producer Greg Amante of ESPN's Enterprise and Investigative Unit contributed to this report.