Monteith, 31, who rose to fame in 2009 for his role as Fin Hudson in Fox sitcom "Glee," was found dead Sunday in a Canadian hotel.
After years of struggling with addictions, the Canadian actor had checked into a rehab clinic in April. Adam Shankman, who directed an episode of "Glee" for each of the past three seasons, told CNN's Poppy Harlow that he had talked to Monteith on Saturday morning and the actor had said he was "doing amazing."
Monteith checked in at the Fairmont Pacific Rim Hotel in Vancouver, Canada, on Saturday. He was found dead by hotel staffers on Sunday afternoon after he missed the check out deadline, Police Chief Doug LePard said.
Authorities added that there was no indication of foul play and that "the cause of death was not immediately apparent." An expedited autopsy will be carried out on Monday as police claimed they needed further tests to determine how the actor died.
"The exact nature of those examinations will depend on investigative findings within the next day or two as information is gathered from medical records and discussions with family take place," said B.C. Chief Coroner Lisa Lapointe.
Footage from the hotel's security cameras show the "Glee" star had returned to his hotel room early Saturday morning after leaving the room with people earlier. He was believed to be alone when he died, Le Pard said.
Lapointe added that Monteith had been dead for several hours when his body was found. People he was with before his death are currently being interviewed by investigators.
In a June 2011 interview with Parade magazine Monteith confessed he was "lucky to be alive" after his many years struggling with addiction.