The document, which was released on Wednesday, revealed that investigators found evidence of drug use as well as two empty champagne bottles in the hotel room where the actor was found.
Cory had checked into the Vancouver Fairmonth Pacific Rim Hotel on 6 July, and when he didn't check out as expected on 13 July, hotel staff entered his room and found him unresponsive on the floor.
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Hotel footage showed him returning alone to his room at 2.16am on 13 July.
The investigation confirmed that the 31-year-old Canadian actor had a history of illicit drug use with intermittent periods of rehabilitation and abstinence from drugs.
"It said that when people who have refrained from using opiates, such as heroin, for a while, their tolerance for the drug decreases.
"Individuals typically build up a concentration tolerance to this type of drug over time. After a period of cessation from opioid drug use, a previously tolerated drug concentration level may become toxic and fatal."
Cory was honoured at the Emmy Awards last month.
In the past, he had spoken openly about his struggles with substance abuse, and admitted in a 2011 interview that he had previously entered rehab when he was 19.
At the time, the actor said he hoped being honest about his battle with addiction would help others.
"I don't want kids to think it's ok to drop out of school and get high, and they'll be famous actors too," he said in 2011. "But for those people who might give up: get real about what you want and go after it."
Glee became an instant hit when it debuted in 2009, and has since featured high-profile guest stars like Britney Spears and Gwyneth Paltrow.
On 10 October, a tribute episode to Cory will air, which producers have already confirmed will deal directly with drug addiction.
Glee creator Ryan Murphy has also said it will feature PSAs – public service announcements – with cast members in character, including Cory's brave girlfriend Lea Michele.