WEATHERFORD, Texas -- Gary Grimm doesn't recall much about the June morning he almost died, even when he watches security footage from the basement of the Parker County District Courthouse.

"I just slumped and went out," he said. "Next thing I remember is looking up at the ceiling in the ER."

What happened between was captured on surveillance video that quickly went viral.

Gary was assigned to monitor Parker County inmates in a holding cell below the courtroom on June 23, 2016.

The lone guard in the room, he was sitting across from a locked gate holding at least eight men when a massive heart attack struck.

"They thought I was kidding," Grimm told WFAA recently. "I wasn't."

Inmates soon realized the guard was unconscious. He had no pulse. They started shouting and banging into the holding cell siding.

Eventually, they broke free and caused enough commotion that deputies from the courtroom came to investigate.

"Instead of overtaking me and taking my gun, and killing me or taking a hostage and escaping, they looked at me as a human being," said Grimm.

Through a 20-plus year working security at Texas prisons and jails, Gary came to treat most inmates the way he would've wanted to be the treated.

"I don't care if they're a drug user, hot check writer, or child molester. They're still a human being. I don't have to like what they did, but it's not my position to be the judge and jury," said Grimm.

The 52-year-old medically retired from his jail gig in December because of ongoing heart concerns.

He's set for open heart surgery this week to get a Left Ventricle Assistance Device, or LVAD, installed.

"I need that for at least six months, and then maybe we can have a transplant," said Grimm.

The man never got a chance to personally thank any of the inmates because they were transported elsewhere while he recovered in the hospital.

But he tears up thinking about what could've happened if they hadn't acted.

"I get emotional. If they wished harm for me, all they had to do was sit there and do nothing," said Grimm.

Friends have started a GoFund Me page to help the family with extra expenses as Gary awaits a possible transplant.