The youngster — named locally as 17-year-old Jack Scorgie — died at a first-floor property on the corner of Maclagan Road and Barry Road, Carnoustie, yesterday.
Ambulance crews raced to the flat after reports that Jack had suffered a cardiac arrest.
He was pronounced dead at the scene.
Officers confirmed today that they were probing the death and treating it as unexplained.
A significant police presence has remained at the flat since the call came in at around 7.40am yesterday.
Elizabeth Low, 60, who lives in the same block where Jack died, described the news as a “tragedy”.
She said: “I’ve known the family for 20 years.
“I didn’t know anything had happened until Sunday morning when I got up and there were all the police vehicles outside.
“I’ve been told it was the young lad, Jack, who has died.
“It’s a tragedy for someone that young to die, it doesn’t matter the circumstances.
“I’ve lived here 20 years but waking up to police outside like that is awful.”
Another neighbour who lives across the road said: “It was strange as I never heard anything happening — I found out on Facebook and that’s when I looked out and saw the police.
“I don’t really know the family but it’s so sad to hear. No one should be burying a teenager, he had his whole life ahead of him.”
A spokesman for Police Scotland said: “Officers attended at Barry Road, Carnoustie, yesterday morning following the sudden death of a 17-year-old male.
“The death is being treated as un-explained and as with all sudden deaths a report will be submitted to the procurator fiscal.”
A spokesman for the Scottish Ambulance Service said: “We received a call at 7.40am yesterday to attend an address in Barry Road.
“We dispatched one ambulance to the scene.”
Tributes have poured in for Jack on social media from friends and family.
Jamie Grant wrote on Facebook that he was “heartbroken”, adding: “We will see you again one day.”
Lynsey Torrie added: “Such a nice wee lad, thinking of you all.”