Cult
Ratings for the current season are down 30% year-on-year, and this week's episode pulled in just 7 million viewers on NBC. In the UK, a documentary called Revenge Of The Bin Men recently beat Heroes in the ratings.
Grunberg, who plays Matt Parkman in the series, added to speculation over the show's future with a message on Twitter.
He wrote: "Tough to say goodbye to crew not knowing if any or all of us will return next year. Hope all."
A few minutes later, Grunberg posted: "Don't get me wrong, Heroes IS coming back next year, but some crew take other jobs, so it's tough... we have the Best. Crew. Ever."
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