Alec Musser died by suicide -- and the nature of his death is gruesome ... TMZ has learned.

The San Diego County Medical Examiner's Office tells TMZ ... the 'All My Children' star was discovered dead last Saturday by his fiancée at their Del Mar, CA home -- and according to the report provided, it says he suffered an apparent self-inflicted gunshot wound.

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We're told Musser's fiancée went to bed on Friday evening, and awoke the next day to find AM slumped over on their bathroom floor ... with a firearm near him as well.

The ME's office goes on to say that 911 was called, and when paramedics arrived -- Musser was confirmed as deceased. They note the firearm that was used was a shotgun ... and his death is listed as a suicide.

TMZ broke the story ... Musser's fiancée, Paige Press, confirmed his passing to us this past weekend, but didn't elaborate on the circumstances at the time. She later took to social media to remember her partner as "the best fiancé I could of ever asked for."

Musser is most famous for playing Del Henry on the soap hit 'All My Children' -- not to mention roles in 'Grown Ups,' 'Desperate Housewives' and other shows here and there.

He was only 50.

RIP

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