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.

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.

Ace was a founding member of KISS … joining forces with Gene Simmons, Paul Stanley and Peter Criss. He shredded guitar with the band from 1973 to 1982 … but left the group amid substance abuse issues and creative differences.

He now holds a bittersweet place in rock history as the first original member of KISS to pass.

Ace was 74.