Brennan tragically died at the age of 37 in May 2021. The former University of Hawaii quarterback had been receiving treatment at a drug rehab facility in Southern California when he experienced a medical emergency. He was rushed to the hospital ... where he passed away.

Now, after completing an autopsy and tox screen, health officials tell us the death was accidental, caused by a lethal concoction of drugs (polydrug), including the dangerous and illicit substance, fentanyl.

In addition to the incredibly powerful opioid, officials also found methamphetamine, amphetamine and ethanol in his system.

It's unclear if any of the drugs were prescribed by doctors ... or if Colt took them on his own.

After starring at Hawaii, Colt was a 6th-round draft pick in 2008. He bounced around the league ... playing with Washington and Oakland, before playing ball with the CFL in Canada.

Brennan was from Newport Beach, California.