Former NFL great Harry Carson has a message for parents: Don't let your kid play football until you see what we've learned about head injuries.

Carson, a Hall of Fame linebacker for the New York Giants, is the star witness for "League of Denial: The NFL's Concussion Crisis," a "Frontline" investigation coming Oct. 8 on PBS. Out of the game for 25 years, Carson says he was diagnosed with post-concussion syndrome that he says resulted from the hard hits he took repeatedly during his playing years. Many retired players have sued the league, claiming that years of injuries led to neurological damage, dementia and other illnesses.

And it's not just about pro football; Carson says injuries start accumulating negative effects when players are young.

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"Parents need to know what they're signing up for," Carson, 59, told reporters Tuesday at the TV press tour in Beverly Hills.

Carson has already decided what the answer will be for his 3-year-old grandson.

"Knowing what I know now, I do not want him to play," he said. "I've told his mom, my daughter, that he's not gonna play football."

Not surprisingly, producers said the NFL refused to cooperate with "League of Denial." But Carson says the public was shaken by the suicide last year of Junior Seau, a former Pro Bowl linebacker. An autopsy revealed that Seau was suffering from a type of chronic brain damage frequently seen in former NFL players. Seau's death was a "heads-up," Carson said.

"Attitudes are changing," he added.

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Twitter: @scottcollinsLAT