"There is no evidence to suggest Mr Monteith's death was anything other than a most tragic accident," the office said in a statement.
The 31-year-old was found dead in his Vancouver, British Columbia, hotel room on Saturday after he didn't check out on time. He was believed to be alone when he died. Hotel video and electronic records indicate he returned to his room by himself early on Saturday.
At a briefing on Tuesday afternoon, police said they believe Monteith had been dead for several hours before he was found. They said the coroner's report didn't indicate the levels of heroin or alcohol in his system. They ruled out foul play.
'I had a serious problem'
"Our belief is that when he took the heroin he was alone," said Vancouver Police Department spokesperson Brian Montague. He added: "There was evidence in the room that was consistent with a drug overdose. We're not providing exactly what we found at the scene."
Monteith's death recalled the lives of Heath Ledger, Corey Haim and River Phoenix - actors who battled substance abuse and died in their 20s and 30s.
Monteith had talked bluntly about struggling with addiction since he was a teenager, calling it a serious problem and telling Parade magazine in 2011 he was "lucky to be alive."
In that interview, he said he was using marijuana and drinking by age 13, and his drug use was "out of control" by the time he was 16. "Anything and everything, as much as possible," he said. "I had a serious problem."
'He looked super fit'
Monteith admitted himself to a treatment facility in April for substance addiction, a representative said at the time. He also received treatment when he was 19. He told Parade that his mother and friends had staged an intervention back then, afraid he "could die." However, he said, "I did the stint but then went back to doing exactly what I left off doing."
Typically, the younger a person gets hooked on drugs or alcohol, the higher the risk of relapse. It's also more challenging for people addicted to multiple substances.
"When an addicted person re-enters their environment, they have a lot to negotiate" such as finding a sober network of friends and not giving in to cravings, said addiction expert Dr Karen Miotto at the University of California, Los Angeles. Such pressures can cause a person to relapse.
Gia Milani, who recently produced and directed a Canadian film featuring Monteith, this week said he "seemed healthy" when she last saw him four weeks ago in Los Angeles.
"He looked super fit and he was energetic and excited," Milani said. She said Monteith showed no signs of a substance abuse problem while shooting the film a year ago.
Glee, with its catchy song-and-dance numbers and high-profile guest stars like Gwyneth Paltrow and Britney Spears, became an instant hit when it debuted in 2009. Monteith served as the show's resident hunk with a heart of gold. The show's producers have called him an exceptional performer "and an even more exceptional person."
The publicist for Monteith's girlfriend and Glee co-star, Lea Michele, released a statement on Tuesday.
"Since Cory's passing, Lea has been grieving alongside his family and making appropriate arrangements with them," it said. "They are supporting each other as they endure this profound loss together." The statement was first reported by People magazine.
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