Image copyright Getty Images Image caption Colin Kroll was co-founder of both HQ Trivia and Vine

The co-founder of the popular app HQ Trivia, Colin Kroll, died of an accidental drug overdose, according to the New York City medical examiner.

A post-mortem examination revealed two kinds of fentanyl, heroin and cocaine in his system.

Police found Mr Kroll dead in his Manhattan flat in December after being asked to check in on him by a woman reported to be his girlfriend.

The 34-year-old was also the co-founder of the video platform Vine.

The medical examiner's office ruled Mr Kroll's 16 December death an accident, due to "acute intoxication" from the combined effects of the drugs.

Fentanyl - a synthetic opioid 80 to 100 times stronger than morphine - and a variant, fluoroisobutyryl fentanyl, were both found in Mr Kroll's system.

Mr Kroll's death was suspected to be due to an overdose after police found his body without signs of trauma and with drug paraphernalia nearby.

He was reportedly found face down on his bed, and police noted signs of what appeared to be cocaine and heroin in the flat.

Mr Kroll's friends and family remembered him as a kind, talented young man.

His former fiancé Maggie Neuwald told the New York Post after his death that he had struggled with the pace of the tech industry.

"It's not like anyone hands you … a manual of how to deal with [success]," she said. "That probably, unfortunately, got the best of him, although I had hoped he'd be able to fight those demons."

Last year, the Centers for Disease Control found synthetic opioid-related overdose death rates had risen by 45% on average across the country in one year.

Image copyright Getty Images Image caption Colin Kroll had won a Breakthrough Award for Emerging Technology in 2014

Mr Kroll had been named CEO of the HQ Trivia mobile app in September. He founded the game with Rus Yusupov.

The live trivia game became hugely popular, although its appeal waned last year. The free app was guest-hosted by some famous faces, including Jimmy Kimmel and Bert from Sesame Street.

Vine was a popular a six-second video streaming service that Twitter purchased in 2012 for $30m (£24m) and eventually discontinued in 2016.