SAN DIEGO -- A guard at a California penitentiary claims prison staff violated an inmate's civil rights but supervisors swept it under the carpet.

"It was malicious; it was intentional," said the guard at Centinela State Prison. He can't be identified for fear of repercussions. "The inmate could have died."

The inmate is Robert Douglas, convicted of shooting a woman to death during a parking lot argument in Compton.

The country's laws grant all inmates civil rights behind bars, and the guard and Douglas said his were violated July 8, 2008.

"They just disregarded all of the rules, did whatever they wanted to," said Douglas.

Douglas said that it started when he and another lieutenant at the prison exchanged words during a search of a prison building.

The prison has two isolation cells, 10News learned, and both were available that day. But two guards confirmed that prison staff put Douglas in a van parked outside. It had no insulation; there were no windows cracked and no ventilation.

Copies from the log book obtained by the 10News I-Team show the temperature that day was 111 degrees.

The guard described Douglas' reaction.

"He was banging on the inside -- 'Let me out; I can't breathe," the guard said.

Inside the van for roughly an hour, Douglas said he was cooked.

"It was so hot, right, I mean, when I came out of the truck, everything was blury," Douglas said.

The guard confirmed Douglas passed out and had to be taken to the infirmary.

Douglas' formal complaint went up the chain of command at Centinela State Prison. In response to questions from the I-Team, a representative wrote that "his claims were unsubstantiated, staff followed all laws, procedures, and policies."

Matt Gray lobbies for prison reform. He said, "Awful, awful things go on in our prison system."

He said what happened to Douglas is not common but still happens too often and people should be outraged.

"This is undoubtedly going to rise to the level of a civil rights violation, which the taxpayer will end up paying for," he said.

The prison guard said high-ranking supervisors witnessed the van incident.

"They didn't take any responsibility for their actions, swept it under the carpet," he said.

A California Office of Inspector General audit found that prison officials, when investigating wrongdoing, fail to "record the findings" 40 percent of the time -- making it hard for the inspector general to gauge what's going on behind bars.

In the last six months of 2008, there were 1,790 incidents of use-of-force by prison guards, including 55 criminal cases against California prison staff.