Bruce Springsteen at Giants Stadium in East Rutherford, N.J. July 27, 2008.
Sullivan, a cousin of the rock veteran, was assistant road manager to Springsteen's E Street Band.
He was found dead at Kansas City's InterContinental Hotel on Oct. 26, just hours before Springsteen and the band were to take the stage at the Sprint Center.
The show was cancelled.
Sullivan, 36, died of acute amphetamine and heroin intoxication according to autopsy results released Tuesday.
The manner of death was ruled an "accident."
"Lenny Sullivan, Bruce's cousin and the assistant road manager of the E Street Band for the last 10 years, passed away today at the age of 36," a statement on Springsteen's Web site read shortly after the news broke. "A warm and sensitive person, he was beloved by Bruce, the Band, the crew, and the entire Thrill Hill family."