"League of Denial: The NFL's Concussion Crisis" preview
Video duration: 2:15
The National Football League, a multibillion-dollar commercial juggernaut, presides over America’s indisputable national pastime. But the NFL is under assault as thousands of former players and a host of scientists claim the league has covered up how football inflicted long-term brain injuries on many players. FRONTLINE investigates: What did the NFL know, and when did it know it?