Legendary KISS guitarist Ace Frehley passed away Thursday, Oct. 16, after his family made the difficult decision to turn off his ventilator … and today his cause of death has been released, TMZ has learned.

TMZ confirms, Ace died as a result of blunt trauma to the head due to a fall.

According to the Morris County, New Jersey Medical Examiner’s report … Ace suffered a fracture to the back of his skull, a subdural hematoma — a collection of blood formed between the brain’s protective outer layer and the brain itself — and a stroke. His manner of death was ruled an accident.

The 74-year-old musician was put on life support after a nasty fall in a studio led to a brain bleed. According to the report, he underwent a procedure to evacuate the subdural hematoma, but to no avail.

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.

Back in March this year, Ace was all smiles talking about his comeback following a nasty arm injury … in an interview with New Jersey newspaper The Aquarian, he said, “I just spoke to a psychic. She goes, ‘Ace, you’re going to live to be a hundred.’ So I said, ‘No problem!'”

He’s the first original member of KISS to die.