The bassist for the Grammy-winning metal band Slipknot died of an "accidental" overdose of morphine and fentanyl, a synthetic morphine substitute, police in Iowa said as autopsy results were released.

Paul Gray, 38, was found dead on May 25 at a hotel near Des Moines, Iowa, the hometown of the band who always appear in public in grotesque masks, beat each other up on stage and call their fans "maggots." Gray co-founded the band in 1995.

An autopsy conducted by the Polk County Medical Examiner's Office also found that Gray suffered from "significant heart disease," according to a statement released by the Urbandale police department.

After Gray's death the eight surviving members of the band shocked fans by appearing in public for the first time without their trademark masks and coveralls to pay tribute to him. They were accompanied by Gray's wife Brenna who is pregnant with their first child.

Slipknot topped the charts in the United States, Britain and Australia with its most recent album, 2008's "All Hope Is Gone." The band won a Grammy in 2004 for its song "Before I Forget."