UFC Hall of Famer Stephan Bonner’s official cause of death is now known.

Bonnar, who was a finalist on the first season of “The Ultimate Fighter,” died on Dec. 22 in Las Vegas at 45. The UFC announced the news two days after, citing heart complications at work. However, the Clark County (Nev.) coroner’s office has ruled Bonnar died due to an accidental drug overdose.

According to a report from MMA Fighting, the coroner’s office officially ruled Bonnar’s death as accidental due to “fentanyl, paraflurofentanyl, and mitragynine intoxication.” No other details were revealed due to medical privacy laws.

UFC president Dana White described Bonnar as “one of the most important fighters to ever compete in the octagon” for his classic UFC Hall of Fame fight against Forrest Griffin in April 2005. The news of Bonnar’s untimely death shook up the MMA community.

According to the U.S. Centers for Disease Control and Prevention, fentanyl is a synthetic opioid so powerful that it is up to 50 times stronger than heroin and 100 times stronger than morphine. It is often found in counterfeit pills and mixed with other drugs such as cocaine, methamphetamine and heroin. Unless tested, it is nearly impossible to detect if street drugs are laced with fentanyl.

In February, U.S. Drug Enforcement Agency administrator Anne Milgram described fentanyl as “the deadliest drug threat our country has ever faced.”

The UFC hosted a Celebration of Life ceremony for Bonnar in February at the UFC Apex in Las Vegas, where a number of speakers paid tribute, including White, his historic rival Griffin, and friends and family.