The NYC Chief Medical Examiner tells TMZ ... Colin died from acute intoxication due to the combined effects of fentanyl, fluoroisobutyryl fentanyl, heroin and cocaine. They've ruled his death accidental.

We broke the story ... the HQ honcho died back in December, when cops found his body lying face down in his bedroom with drug paraphernalia nearby.

As we reported ... Kroll's GF told investigators she was with him the Friday night before he was found dead in his Manhattan apartment, and they were both doing drugs. His GF has not been charged in connection with his death.

Colin co-founded the popular mobile trivia game in 2017 and was elevated to CEO in 2018. Kroll was also the co-founder of Vine and was General Manager between 2013 and 2014. He previously worked for Twitter, Jetsetter and Yahoo.

Kroll was 34.

Fentanyl also played a role in the deaths of Tom Petty, Prince, Mac Miller and Lil Peep.