Rialto police said the county medical examiner's report showed King had drugs such as PCP, cocaine and marijuana in his system when he died in June, the San Bernardino Sun reported.
The autopsy report is due to be released later Thursday.
"There's going to be no surprises. His death was ruled an accidental drowning," said Rialto police Capt. Randy DeAnda.
King also had a blood-alcohol level of .06 percent.
"The investigation is now completed and basically the autopsy and toxicology reinforces our conclusions," DeAnda said.
A call from King's fiancee brought police officers to King's home in Rialto at 5:30 a.m. on June 17. The officers pulled the 47-year-old King from the bottom of his pool and he was pronounced dead at a hospital.
King was the black motorist severely beaten by Los Angeles police officers in 1991. The officers' acquittal on beating charges triggered the devastating 1992 Los Angeles riot.