KTLA's Sam Rubin's Cause Of Death Confirmed by Medical Examiner

Sam Rubin had a massive blockage of his coronary artery ... according to the L.A. County Medical Examiner's report on his death.

The M.E.'s Office says it determined KTLA's famed entertainment reporter officially died of ischemic heart disease due to atherosclerotic coronary artery disease. Translation: Sam suffered a heart attack due to the blockage.

As TMZ first reported ... Sam was rushed to UCLA West Hills Hospital back in May, and we were told he had gone into full cardiac arrest while at home.

Sam had been with KTLA since 1991, and was a mainstay in Southern California homes for decades, covering entertainment, movie, and TV news ... winning multiple Emmys along the way.

He's survived by his wife, Leslie, and his 4 children.

Sam was 64.