Image caption The band, with Gray (far right) won a Grammy Award in 2006

The death of Paul Gray, bassist for US metal band Slipknot, was due to an accidental overdose, an autopsy shows.

Toxicology tests showed a fatal level of morphine and fentanyl, a synthetic morphine substitute, in his blood. Police said there was no evidence of a prescription for either drug.

His body was found by an employee at a hotel in Des Moines, Iowa, on 24 May.

The band, famous for their ghoulish masks, won a Grammy Award in 2006 for the song Before I Forget.

The autopsy also revealed signs of significant heart disease, police in Urbandale, a Des Moines suburb, said.

A spokesman for the Urbandale police said they were trying to find out how Gray obtained the drugs.

The spokesman said charges were possible.

The band formed in 1995 in Des Moines and remained based in the city.

The nine members of Slipknot wore Halloween masks in public and referred to other bandmates by numbers - Gray was number two.