Friends and colleagues have expressed shock at the death of Glee star Cory Monteith, who they said seemed fine in recent weeks.
The 31-year-old actor was found dead at Fairmont Pacific Rim hotel in Vancouver just after noon on Saturday.
Adam Shankman, a director on Glee, told CNN: "I had several interactions with him yesterday where he said that he was doing amazing.
"He even said 'I am feeling fantastic'.
Monteith checked into rehab in April
"I'm like everybody else, really devastated and confused by what happened."
The Canadian actor had been dead for several hours when his body was discovered after he failed to check out of his hotel.
There were no signs of foul play and the cause of his death was not clear, British Columbia Chief Coroner Lisa Lapointe said.
A post-mortem examination is expected to take place later today.
Monteith's representatives issued a statement saying: "We are in shock and mourning this tragic loss."
X-Factor judge Sharon Osbourne told Sky News of her devastation at the news. She said: "It broke my heart, what can you say? A beautiful, talented young man. Unfortunately, the demons in his head got to him, it's very, very sad."
The actor 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.
The actor's cousin Richard Monteith at a memorial outside the hotel
Asked whether Monteith had died of a drug overdose, Vancouver Police Department Acting Chief Doug LePard said he would "not discuss anything that we might have found in the room at this time".
Other people were in Monteith's hotel room on Friday night but evidence, including fob key entries, showed he returned to his room by himself early on Saturday and was alone when he died, police said.
Mr LePard said the people who were out with Monteith before he died are being interviewed.
Monteith began his career in a number of small TV and film roles, but the hit musical show Glee was his breakthrough.
His character's on-again-off-again romance with Rachel Berry, played by Lea Michele, was a big part of the show.
Monteith and Michele followed in the footsteps of their Glee characters by dating in real life.
They appeared publicly together as recently as last month at a charity ball.
Monteith and Lea Michele in June
Michele has asked for privacy after receiving the news of her boyfriend's death.
"We ask that everyone kindly respect Lea's privacy during this devastating time," said her representative, Molly Kawachi.
When Monteith entered rehab in April, Michele told People magazine she loved and supported him and was proud he was seeking help.
His cousin Richard Monteith cried at a makeshift memorial outside the hotel where the actor died, while stars, co-workers and fans have been expressing their sadness on Twitter.
Monteith's Glee co-star and close friend Matthew Morrison dedicated his performance of 54 Below at a New York Broadway nightclub to the actor on Sunday evening, describing him as a "brother".
According to the New York Times, an emotional Morrison told the audience: "You guys came to see a show. I came to perform a show, so that's what we'll do. And we'll do it in Cory's honour."
He changed his opening song to 'What I did for Love' from the musical A Chorus Line, which featured in the second season of Glee.
Monteith was an avid supporter of Project Limelight, a Vancouver charity that offers a theatre and arts programmes to at-risk youth.
He dined with Project Limelight co-founder Maureen Webb at a Vancouver restaurant just days before his death.
In an interview last year, Monteith credited Ms Webb for suggesting that he enrol in acting classes when he was struggling at 19.
Monteith on stage with Lady Gaga and Glee co-star Jane Lynch in 2010
He kept in touch with Ms 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."
A Glee convention taking place near London's Heathrow airport at the weekend turned into a memorial for Monteith after attendees heard of his death.
Organisers of the three-day event changed the final day's schedule so fans could mourn the star together.
Chloe-Louise Bond, a 22-year-old fan from Wakefield, said: "We all woke up to hear the story, and no one really wanted to believe it was true.
"Walking into the main room, you could just feel the tragedy in the air, absolute strangers became a family right in that moment. Everyone was crying and hugging and just trying to get over the shock."
The fans chanted "Cory! Cory! Cory!" and sang in unison tunes such as Don't Stop Believin', the Journey cover crooned by Monteith in the first episode of Glee.
Curt Mega, Telly Leung and other actors who have appeared in the programme led a group discussion with fans about their memories of Monteith.