OKLAHOMA CITY (AP) — State and federal prisoners in Oklahoma are among the most likely to be killed or die accidentally behind bars, according to a new federal study.

The figures released earlier this month by the U.S. Bureau of Justice Statistics show Oklahoma had the second-highest prison homicide rate in the country from 2001 to 2014, with 13 killings per 100,000 state and federal inmates. Oklahoma's rate was more than double the national average of five per 100,000 inmates and second only to Maine's rate of 14 per 100,000, but the study warned Maine's figures were unreliable because of a small sample size.

"We know that we've truly become just a warehouse for inmates in Oklahoma. We don't really offer them any programs to rehabilitate them. We barely staff our facilities. Our facilities are crumbling and falling apart," said Sean Wallace, policy director for the Oklahoma Public Employees Association and the former head of a group that represented prison workers. "I doubt there's one person in the Oklahoma Department of Corrections who is very surprised that we rank high in the number of inmates who died in our prisons."

The study also showed Oklahoma had an accidental death rate of eight per 100,000 inmates, nearly triple the national average and second only to Alaska, which also had unreliable data.

Overall, Oklahoma's annual mortality rate of 324 inmates per 100,000 from 2001 to 2014 was tied for sixth in the nation with Mississippi. Louisiana was first with 477 deaths, followed by West Virginia, Tennessee, Kentucky and Arkansas. The national average was 255 deaths per 100,000.

Oklahoma prisons spokesman Alex Gerszewski said it's difficult to compare state-to-state numbers on prison deaths because not every state uses the same method to record deaths. But the BJS report provides a detailed accounting of its methodology and instructions provided to prison officials in all 50 states.

The figures include all deaths that occur when an inmate is in the custody of a state or federal prison, including private prisons or any state-operated facility. Deaths that occur in jails are not included, and neither are deaths of inmates on escape status or under the supervision of community corrections, like probation, parole or home electronic monitoring.

Gerszewski said the general poor health of inmates and a dramatic increase in the number of aging inmates likely play a role in Oklahoma's high mortality rate. Over the years the Legislature has increased the number of so-called 85-percent crimes, which require an inmate to serve at least 85 percent of his or her sentence before becoming eligible for parole. This has led to an increase in the number of older inmates.

Of the 109 state prison deaths recorded in 2015, 72 of the inmates were older than 50, Gerszewki said.

"Inmates coming into the system oftentimes have pre-existing medical conditions that put their health in a compromised state," Gerszewski said. "The department does its best with the resources available to care for them and provide treatment."

Wallace said attempts to pass legislation in recent years to allow for aging inmates to receive some kind of expedited medical parole have faced resistance in the Legislature. He said he expects the OPEA will push for a similar proposal next session.