Twisted Sister Drummer A.J. Pero ... Dead at 55 After Being Found Unresponsive on Tour Bus

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A.J. Pero -- drummer for the legendary rock band Twisted Sister -- died Friday morning after he was found unresponsive on a tour bus in Poughkeepsie, New York. He was 55.

Jay Jay French, a fellow member of Twisted Sister, tells TMZ ... Pero was touring with the band Adrenaline Mob when he was found in the bus, but would not wake up. The band called 911 and paramedics rushed him to a nearby hospital ... but he didn't make it.

French tells TMZ, "A.J. Pero was a force, the likes of which I doubt I will ever have the chance to play in front of again. His drumming was legendary, but his heart, as a human being, was even greater."

He continued, "In our hearts he will always be the engine that drove the train. I loved him and today my heart has been ripped out. RIP."