Mark Fainaru-Wada and Steve Fainaru discuss the new Frontline documentary “League of Denial: The NFL’s Concussion Crisis,” which they reported and wrote. Thousands of former players have claimed the National Football League tried to cover up how football inflicted long-term brain injuries on many players. The documentary asks: What did the NFL know, and when did it know it? Mark Fainaru-Wada and Steve Fainaru are also the authors of a forthcoming book League of Denial. “League of Denial: The NFL’s Concussion Crisis” airs October 8, 9-11 P.M. on PBS.