UFC legend Stephan Bonnar died from accidental fentanyl overdose, officials say

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Hall of Fame fighter Stephan Bonnar, who played a significant role in the development and growth of UFC, died in December at the age of 45. 

More than four months after his unexpected death, officials have released a cause of death. 

Stephan Bonnar poses with his UFC Hall of Fame plaque during the UFC Fan Expo Las Vegas 2013 at the Mandalay Bay Convention Center in Las Vegas on July 6, 2013. (Al Powers/Zuffa LLC/Zuffa LLC via Getty Images)

The UFC announced shortly after his death in late December that Bonnar had died from "presumed heart complications while at work." 

FIGHTERS REACT TO STEPHAN BONNAR’S SUDDEN DEATH AT 45: 'CHANGED THE LANDSCAPE OF THE SPORT'

But the Clark County coroner’s office in Las Vegas confirmed to MMA Fighting on Tuesday that Bonnar died from an accidental fentanyl overdose. 

According to the report, Bonnar died from "fentanyl, parafluorofentanyl and mitragynine intoxication" on Dec. 22. 

Mark Coleman, left, takes on Stephan Bonnar during UFC 100 at Mandalay Bay Events Center in Las Vegas on July 11, 2009. (Josh Hedges/Zuffa LLC via Getty Images)

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"Stephan Bonnar was one of the most important fighters to ever compete in the Octagon," UFC president Dana White said in a statement at the time. "His fight with Forrest Griffin changed the sport forever, and he will never be forgotten. The fans loved him, related to him and he always gave them his best. He will be missed."

Largely considered one of the pioneers of UFC, Bonnar was inducted into the sport’s Hall of Fame in 2013. 

Stephan Bonnar weighs in before UFC 153 at HSBC Arena in Rio de Janeiro on Oct. 12, 2012. (Josh Hedges/Zuffa LLC/Zuffa LLC via Getty Images)

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He fought to a 17-9 record and had battles with UFC legends Jon "Bones" Jones, Anderson Silva and Tito Ortiz — the latter two fighters were his final opponents.

According to MMA Fighting, Bonnar opened up about his use of opioids in a 2021 interview with the outlet in which he claimed he was properly using them to treat long-term injuries suffered as a result of his career.

Fox News’ Ryan Morik and The Associated Press contributed to this report.