According to TMZ, health officials have determined his death was due to an accidental overdose.
"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," the TMZ report reads.
"In addition to the incredibly powerful opioid, officials also found methamphetamine, amphetamine and ethanol in his system."
Back in May, Brennan's father Terry hinted at this conclusion when he told ESPN his son "ingested something laced with fentanyl and never regained consciousness."
At the time of his death, Brennan was going through a drug rehabilitation program in California.
"He was really into it," Terry Brennan said back in May. "It involved a lot of physical activity and he liked it. He was working with soldiers who had come back from Afghanistan and Iraq with similar problems. He was doing quite well with it for four months. Then something happened and he went to the dark side, and it was just not good."
Our thoughts and prayers remain with Brennan's family and friends as they continue to mourn his passing.