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KISS guitarist Ace Frehley passed away last week after a tragic fall ... and a New Jersey medical examiner is now investigating his death, TMZ has learned.

A rep for the Morris County Medical Examiner tells us an autopsy was not performed on Frehley ... but the rock legend is receiving a toxicology screening along with an external body examination. The rep also said Ace's cause of death will be finalized in the next few weeks after the toxicology report is completed. Gallery Launch Gallery Getty

We broke the story ... Ace was placed on life support after being hospitalized with brain bleed. Ace suffered the bleed following a bad fall in the studio a couple of weeks prior.

Later, the family ultimately made the heartbreaking call to remove his ventilator. Getty

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