Prison officials dispute those claims, saying that the health and well-being of the inmates are their top priorities and that the autopsies of the 12 inmates who died list a variety of contributing factors to their deaths. They said they take steps to help inmates on hot days, including restricting outside work activities and providing extra water and ice.

“It is unknown whether the lack of air-conditioning was a contributing factor in the offender deaths,” Jason Clark, a prison system spokesman, said in a statement. “Texas experienced one of the hottest summers on record in 2011. It was an unprecedented event affecting the entire state and much of the southern and eastern United States. T.D.C.J. took precautions to help mitigate the impact of temperature extremes on offenders and staff. The agency continues to do so.”

But a corrections supervisor who works at a prison where one of the 12 inmates died said the number of heat-related fatalities was a cause of concern, as was the larger number of inmates and corrections officers who require medical attention because of the heat.

At least 17 prison employees or inmates were treated for heat-related illnesses from June 25 through July 6, according to agency documents. Many of them had been indoors at the time they reported feeling ill.

At the Darrington Unit near Rosharon on June 25, a 56-year-old corrections officer fainted in a supervisor’s office and was taken to a hospital. Heat exhaustion was diagnosed. At the four-story Coffield Unit near Palestine, where one inmate died of hyperthermia last August, dozens of windows have been broken out — prisoners slip soda cans or bars of soap into socks and throw them at the windows, hoping to increase ventilation.

“I’m supposed to be watching them, I’m not supposed to be boiling them in their cells,” said the corrections supervisor, who declined to be identified because he was not authorized to speak to the news media. “If you’ve got a life sentence, odds are you’re going to die in the penitentiary. But what about the guy who dies from a heat stroke who only had a four-year sentence? His four-year sentence was actually a life sentence.”

One of the 10 inmates who died last summer, Larry Gene McCollum, 58, a prisoner at the Hutchins State Jail outside Dallas, had a body temperature of 109.4 degrees. Nine days before his death in his cell, the indoor temperatures at Hutchins were routinely recorded by prison officials and ranged from 100 degrees to 102 degrees, according to agency documents.