By the time police arrived several hours later, there were no signs of a struggle and no evidence of foul play. Police were sure, almost immediately, what had happened.
"There was evidence in the room that was consistent with drug use," Const. Brian Montague, of Vancouver police, said at a news conference Tuesday after the B.C. Coroners' Service announced the results of an autopsy and toxicology tests.
"It was the opinion and belief of our investigators at that time that this was going to be a drug overdose."
The service's Barb McLintock said the coroners' investigation will continue. Police say the unanswered questions McLintock's office will determine include the levels of alcohol and heroin in Monteith's system and whether the 31-year-old was the victim of a bad batch of heroin, something which turns up from time to time in Vancouver.
"There's absolutely nothing, no evidence to suggest this is anything other than the most sad and tragic accident," McLintock said in a news release, noting the coroners' service will not comment further until the final report is complete.
Montague said the police investigation is finished. He said the two women and another man Monteith were with last Friday night co-operated fully with police and investigators believe they know exactly what the group was doing and where they were. He wouldn't elaborate and said officers won't pursue where Monteith might have gotten the heroin.
Monteith played the role of football player and singer Finn Hudson on the popular television series "Glee." Before becoming an actor, he worked as a Wal-Mart people greeter in Nanaimo, B.C., as well as a taxicab driver, school bus driver, and roofer.
He also played a drummer for the California-based band Bonnie Dune, and was an avid supporter of the Project Limelight Society, a Vancouver charity that offers a theatre program to at-risk youth. His family has requested donations in his name be made to Project Limelight and two other charities.
On his Twitter feed, Monteith describes himself as a "tall, awkward, Canadian, actor, drummer, person."
For days, a makeshift memorial of cards and flowers has been visited by fans, friends and at least one relative outside the hotel where he died.
Monteith had spoken publicly in the past about his struggles with drug addiction and had reportedly entered rehab earlier this year, but the role of drugs in his death still came as a shock to his fans.
"Just hearing about it, I was shocked," Tyler Gibbs, 21, said from the sidewalk shrine. "It wasn't something I expected. I hear he just completed rehab ... and all of sudden this happens. I can't even come up with words to describe it."
"I didn't expect to hear about the heroin at all. That blows my mind."
Marcela Zuniga, 17, said she simply couldn't believe the news at first, but was left deeply saddened by the cause of Monteith's death.
"I've known many people who have fallen to addiction, and I just stand by my views that I think when you do drugs you're sort of being controlled by demons, so he just fell to that."
Added Shirley Sadler, who broke into tears: "It's just something he got into. It's hard to get out."
Sadler said her nephew died because of an addiction he couldn't kick.
"He tried and tried and tried and he couldn't."
Industry Minister James Moore, who represents B.C. in cabinet, tweeted Tuesday he hoped the "sad death" prompts "a discussion of the complex health issues of addiction and recovery."
Dr. Evan Wood, who works in addiction treatment at St. Paul's Hospital in Vancouver, said a study by the Centre for Urban Epidemiological Studies suggested accidental overdose deaths rose substantially when opiates and alcohol were combined.
The study, published in the journal Addiction several years ago, focused on deaths in New York between 1990 and 1998.
"It's essentially the toxic effect of two depressant substances in combination, being really hazardous," said Wood, who is also a professor of medicine at the University of British Columbia, where he holds the Canada research chair in inner-city medicine
He said the substances work on two different parts of the brain and shut down a person's respiratory centre in the brain until their breathing starts to slow, then stops.
"You'd be under the effects of the drugs to the point where you wouldn't be aware or be able to call for help because you'd be sedated."
Such overdoses often happen when people are alone and have abstained from using drugs for a while, Wood said.
"A very dangerous time for people to overdose is when they've come out of treatment," he said. "Certainly the combination of alcohol and opiates is just a huge risk.
"If he was alone and it appears he was using alcohol, those would be two dangerous combinations but of course the stigma and the shame around addiction often has that unintended consequence of driving people into an environment where they might be alone."
Heroin in Vancouver, often called China White, is typically strong and comes from South East Asia, compared to Los Angeles, where the drug tends to originate from Latin America and has the street name Black Tar, said Wood.
Aiyanas Ormand, with the Vancouver Area Network of Drug Users, a group that advocates for better public policy around illicit drugs, said he couldn't talk specifically about Monteith's death.
But he said VANDU has been lobbying for over a decade for city officials to test illegal drugs and release the findings about potency and what additives might be in them.
"We don't think that people should have to die because they're addicted to drugs and aren't sure of what they're injecting into their bodies."
Ormand said the result of the drug war in a criminalized drug market is that people often don't know what they're getting.
"Occasionally we'll get a health warning — and it's usually after there's been a bunch of deaths — that there's extremely pure heroin on the streets."
Ormand said seasoned heroin users would "taste" the drug first, meaning they would inject a small amount and get a sense of it.
"Often where you get people OD-ing it's with less experienced users or folks who have been in jail or detox and their body was once accustomed to much larger doses ... it can be deadly," he said.
He said it's hard for people to have a sense of quality of the drug when they don't have access to a regular dealer or when they make their purchase in a back alley.
A candlelight vigil for Monteith is planned for this Friday outside the Fairmont Pacific Rim. Before the coroners' announcement, invitations to the evening were spreading on websites and blogs across the Internet.
A separate vigil is also being planned for friends and fans of the actor on Friday at Maple Leaf Square in Toronto.
-- with a file from Camille Bains