(CNN) -- Slipknot bassist Paul Gray's death two months ago was from an accidental morphine overdose, the Polk County, Iowa, medical examiner said Monday.

Gray, 38, was found dead by staff at a hotel near Des Moines, Iowa on May 24, said Sgt. Dave Disney of the Urbandale Police Department.

Toxicology test results showed a fatal level of morphine and fentanyl -- a synthetic morphine -- in Gray's body, said Dr. Gregory Schmunk.

Schmunk also noted the presence of significant heart disease, according to a police statement Monday.

The autopsy did show that Gray had been dead for several hours before his body was found, Schmunk said.

Grammy-winning metal band Slipknot -- whose fans are affectionately known as "maggots" -- was formed in Des Moines in late 1995 and became immediately recognizable for the dark and often grotesque homemade Halloween masks its nine members wore on stage. The band won a Grammy award in 2006 for best metal performance for "Before I Forget."

Slipknot's self-titled debut album went platinum in 2000. Its fourth album, "All Hope is Gone," spent 69 weeks on Billboard's Top 200 charts, debuting at No. 1 its first week in 2008.

Gray was arrested in Iowa in 2003 on drug possession charges when police found small amounts of marijuana, two syringes and a substance that field-tested positive for cocaine after he was involved in a wreck with another motorist.

CNN's Denise Quan contributed to this report.