Cause of death revealed for "Happy Days" co-star, Erin Moran.

Erin Moran who played Joanie Cunningham on "Happy Days" -- likely died from complications of advanced cancer.

The Harrison County Sheriff's Office said Monday a "joint investigation into Mrs. Moran's death was conducted" and "a subsequent autopsy revealed that Mrs. Moran likely succumbed to complications of stage 4 cancer."

The Sheriff also said "standard toxicology tests were performed and the results are pending, however no illegal narcotics were found at the residence."

Erin was found dead Saturday at her New Salisbury, Indiana residence after someone called to report about an "unresponsive female."

She was 56.

Source: TMZ