Police said Sunday that an autopsy is expected to take place Monday to determine the cause of death. Acting Vancouver Police Chief Doug LePard said late Saturday there was no indication of foul play.
The Canadian-born actor, who played Finn Hudson on the Fox TV series about a high school glee club, was found dead around noon Saturday in his room on the 21st floor of the Fairmont Pacific Rim Hotel on Vancouver's waterfront, according to police.
Monteith had openly talked about struggling with addiction since he was a teenager, saying he had a serious problem and took just "anything and everything." He told Parade magazine in 2011 that he was "lucky to be alive."
In April, Monteith checked himself in to a treatment facility for "substance addiction" and asked for privacy as he took steps toward recovery, a representative said at the time. It was not his first time in rehab. He received treatment when he was 19.
Lea Michele, his "Glee" co-star and real-life girlfriend, told People magazine at the time that she loved and supported him and was proud he was seeking help.
Michele was requesting privacy after receiving news of Monteith's death, said her representative, Molly Kawachi of ID-PR .
"We ask that everyone kindly respect Lea's privacy during this devastating time," Kawachi said in in an email to The Associated Press.
Monteith's body was found by hotel staff who entered his room after he missed his check-out time, LePard said. Monteith had checked into the hotel on July 6.
"We do not have a great deal of information as to cause of death," said British Columbia Chief Coroner Lisa Lapointe. She said further tests would be needed to determine how Monteith died.
LePard said Monteith had been out with people earlier and that those people are being interviewed.
Video and electronic records from the hotel indicate Monteith returned to his room by himself early Saturday morning, and he was believed to be alone when he died, LePard said.
Lapointe said he had been dead for several hours by the time his body was found.
Several "Glee" cast members took to Twitter to express their feelings.
"I have no words! My heart is broken," Dot-Marie Jones, who plays football coach Shannon Beiste, said in a post on her Twitter account Saturday night. She called Monteith a "hell of a friend" and an "amazing" man.
Noah Puckerman actor Mark Salling said Sunday he was "going through a million memories and emotions today."
Lauren Potter, who plays Becky Jackson, the cheerleader with Down Syndrome, tweeted that she feels "totally heartbroken right now."
"I love Cory so much this hurts my heart," she wrote. "I hope my Glee family is OK right now. I love them all. Cory was always so nice to me. I have so many good memories."
Harry Shum Jr., who portrays dancer Mike Chang on the show, expressed disbelief on Twitter.
"This tragic news still doesn't seem real to me," said Shum. "I love you, Cory."
John Stamos, who appeared as a guest star on several episodes of "Glee" tweeted: "Heartbreaking. RIP Cory. We talked about how lucky he felt to be alive-and sober. We talked about playing drums. Glad i knew you Cory." Monteith played drums in a California-based indie rock band Bonnie Dune.
Matthew Morrison, who plays glee club coach Will Schuester, issued condolences before performing at a Sunday afternoon show at the cabaret 54 Below in New York, according to the New York Times. The newspaper said he changed his opening number to sing an a cappella rendition of "What I Did for Love" from the musical, "A Chorus Line," which was featured during the second season of "Glee."
"You guys came to see a show, I came to perform a show, so that's what we'll do," he said, according to the newspaper. "And we'll do it in Cory's honor."
The Times said Morrison went on with a set of show tunes and jazz standards, dancing on stage and quipping with the audience about the infamous drug-den history of the space under the 1970's club, Studio 54, where the performance took place.
Fox and the producers of "Glee," including 20th Century Fox Television, called Monteith an exceptional performer "and an even more exceptional person. He was a true joy to work with and we will all miss him tremendously."
"We are in shock and mourning this tragic loss," his representatives at Viewpoint Public Relations in Los Angeles said in a statement.
Monteith, who turned 31 on May 11, starred in "Glee" as a high school football player who puts his status and popularity at risk to join the glee club and its outcast members.
The show, with its pop music-based song-and-dance numbers and high-profile guest stars including Gwyneth Paltrow, became an immediate hit and made stars of its relatively unknown cast.
The series, which debuted in 2009, is in its fourth season.
On his Twitter account, Monteith described himself as "tall, awkward, canadian, actor, drummer, person."
He had recently shot a Canadian film called "All the Wrong Reasons," slated to be released later this year with actors Kevin Zegers, Karine Vanasse and Emily Hampshire.
In a 2010 interview with The Associated Press, Monteith was upbeat about life. He said that if "Glee" were to be canceled he would be OK.
"I've never been afraid of working," he said. "I've never been afraid of auditioning for jobs. Obviously, I've never been afraid of anonymity. I was happy (before 'Glee'). I'm happy now. I guess I'm well adjusted."
Monteith was among the "Glee" actors who remained series regulars as their characters graduated high school and moved on to other adventures. However, in real life Montieth dropped out of high school in Vancouver after his parents divorced.
According to his biography on Fox's website, Monteith was born in Calgary, Alberta, and moved to Vancouver Island as a child. Before turning to acting, he held a variety of jobs including Wal-Mart greeter, school bus driver, roofer and cab driver.
"Thanks for always being kind Cory. You came a long way from hanging on the beaches in Vancouver with the gang pre-Glee," tweeted Gerard Funk, an actor from Vancouver who joined the "Glee" cast last year.
Monteith's TV credits included roles on the series "Kaya" and "Kyle XY" and guest appearances on "Smallville," ''Supernatural," ''Stargate," ''Flash Gordon" and "Interns." His film credits included "Final Destination 3," ''The Invisible," ''Deck the Halls" and "Whisper."
His big break came when he submitted an audition tape to the "Glee" producers in Los Angeles and then raced straight down the Pacific Coast in his car from Vancouver when he was invited to meet them in person.
"I nailed it. I knew I nailed it," Monteith recalled about his audition in an interview with Canadian TV columnist Bill Brioux. "When you walk in to the room, the heads of Fox ... of course it's a little unnerving. Then you realize they're all waiting for you. It's kind of flattering in a way."
Monteith was an avid supporter of Project Limelight, a Vancouver charity that offers a theater and arts programs to at-risk youth. He dined with Project Limelight co-founder Maureen Webb at a Vancouver restaurant just days before his death.
In a Globe and Mail interview last year, Monteith credited Webb for suggesting that he enroll in acting classes when he was 19 years old and going down a "very dark path."
He kept in touch with Webb and made a video to support Project Limelight when the charity was launched last year.
"I think kids really need a place to go and feel like they belong," Monteith said in the video posted on Project Limelight's website. "When I was a kid, I struggled a lot with who I was and where my life was going and what I was interested in. And I was fortunate to have the arts inspire me."
Outside Vancouver hotel, fans paid tribute to Monteith by leaving notes, flowers and stuffed animals.
Helen Slater, 16, who was visiting from England, said she was shocked to hear about his death as she placed a note and a stuffed moose because he had once been photographed wearing moose antlers. She called him a "positive influence" and said "Glee" helped change her life.
"Nothing's going to be the same anymore," she said. The show "just put me (through) some really tough times, and helped me through depression a lot."
Elber reported from Los Angeles. Associated Press writers Charles J. Gans and Frazier Moore in New York and Derrik J. Lang in Los Angeles contributed to this report.