Hastings, 33, died in a fiery car accident in the Hancock Park area of Los Angeles near Melrose and Highland avenues on June 18. A number of conspiracy theories had emerged online following Hastings' death, but the coroner's office classified the death as accidental.
A witness told a photographer that the car was going about 100 miles per hour shortly before the crash.
According to the report, Hastings died instantly of "massive blunt force trauma" the same morning his family was due to arrive in town to convince him to go to detox.
The report states that after 14 years sober, he was using drugs again in the last month after moving to California from New York.
The autopsy found traces of amphetamine in his body, which indicates that Hastings may have taken methamphetamine many hours before his death. Marijuana was also found in his body.
Hastings was hired by BuzzFeed last year, and he was in Los Angeles setting up an L.A. bureau for BuzzFeed.
Hastings, who was also a contributing editor for Rolling Stone, is best known for his 2010 Rolling Stone cover story, "The Runaway General." The story is credited with ending Gen. Stanley McChrystal's career after it revealed the military's candid criticisms of the Obama administration.
The Associated Press and City News Service contributed to this report.