Butler, 33, was discovered dead in his bed at the Mondrian Hotel in Hollywood Dec. 4. His death was ruled accidental.

The Los Angeles Times said the coroner's report stated Butler's death was "due to promethazine/codeine effects and other unestablished factors."

The Texas-born recording artist, who was half of the rap duo UGK, is nominated for a Grammy Award this year.

Ed Winter, assistant chief of the coroner's office, said the amount of cough syrup in Butler's blood was elevated, but not enough to be considered an overdose.

Winter told the Times Butler also had suffered from sleep apnea, which causes people to stop breathing for short periods of time as they slumber.

Coupled with the sleep apnea, the cough syrup likely suppressed Butler's breathing long enough to kill him, Winter said.