Colin Kroll, who co-founded both the Trivia HQ mobile game and Vine app, was found dead in his home in New York City on December 16, Sunday. He was 35.

According to The Hollywood Reporter, NYPD sources said that the cause of death was apparently due to an overdose. However, official reports have yet to be released.

Kroll co-founded Vine with Rus Yusupov in 2012, but it was later acquired by Twitter before the app even launched. In 2017, they released the HQ Trivia mobile game for the iOS and Android. The interactive trivia quiz app takes the form of live shows hosted by many popular celebrities like comedian Scott Rogowsky, Jimmy Kimmel, and more.

Players could win money by participating in the daily live games in HQ Trivia if they correctly answer multiple-choice questions. Only 10 seconds are allocated to each question, which also increases in difficulty as the game goes on.

However, HQ Trivia has since waned in popularity, going from its peak of millions of concurrent players to currently pulling around hundreds of thousands for each game at best. A spinoff game, HQ Words, was also announced earlier in September 2018, with the game currently in semi-public beta at the time of writing.

Fellow HQ Trivia and Vine co-founder Rus Yusupov paid a heartfelt tribute to the late Kroll, who he refers to as his friend in his tweet on Sunday:

So sad to hear about the passing of my friend and co-founder Colin Kroll. My thoughts & prayers go out to his loved ones. I will forever remember him for his kind soul and big heart. He made the world and internet a better place. Rest in peace, brother.

— Rus (@rus) December 16, 2018