The NFL's retirement board awarded disability payments to at least three former players after concluding that football caused their crippling brain injuries -- even as the league's top medical experts for years consistently denied any link between the sport and long-term brain damage.

The board paid at least $2 million in disability benefits to the players in the late 1990s and 2000s, documents obtained in a joint investigation by ESPN's "Outside the Lines" and PBS' "Frontline" show. The approvals were outlined in previously unpublished documents and medical records related to the 1999 disability claim of Hall of Fame center Mike Webster.

The board's conclusion that Webster and other players suffered brain damage from playing in the NFL could be critical evidence in an expanding lawsuit against the league filed in the U.S. District Court in the Eastern District of Pennsylvania. The lawsuit, which involves nearly 4,000 former players, alleges that the NFL for years denied the risks of long-term brain damage and "propagated its own industry-funded and falsified research to support its position."

ESPN reporters Mark Fainaru-Wada and Steve Fainaru are writing a book about football and brain injuries, scheduled to be published in 2013 by Crown Archetype, a division of Random House. PBS' "Frontline," in partnership with ESPN's "Outside the Lines," is producing a documentary based on the reporters' research. This article is a product of these partnerships.

Bob Fitzsimmons, a Wheeling, W.Va., lawyer who represented Webster in his disability case and is co-director of the Brain Injury Research Institute, described the retirement board's conclusions as "the proverbial smoking gun."