Sept. 12, 2015: A California Department of Corrections and Rehabilitation inmate work crew builds a containment line ahead of flames from the Butte Fire, near Sheep Ranch, Calif. (AP Photo/Rich Pedroncelli)

SACRAMENTO, Calif. –  About 1,400 of the state's 3,700 inmate firefighters have previous convictions for violent offenses, California corrections officials said Wednesday, a stunning acknowledgment from an agency that for years said only nonviolent prisoners were allowed in the program.

The disclosure came two days after The Associated Press reported that the corrections department was considering expanding the criteria for inmate firefighters to include those with some violent convictions and with more time left to serve on their sentences.

Corrections spokesman Jeffrey Callison said the department provided inaccurate information to AP and other news outlets and on its website. He said inmates with violent backgrounds have been serving since at least the 1990s but he and other spokespeople did not know that.

The head of the union that represents state firefighters called for an investigation.

Callison blamed the misstatements on differing definitions of what constitutes a violent background. Although the penal code includes hundreds of offenses considered to be violent, prison officials have long considered inmates to be nonviolent if they have a minimum-security classification for good behavior and a significant length of time in prison without committing a violent act.

Arsonists, kidnappers, sex offenders, gang affiliates and those serving life sentences for murder and other crimes have always been excluded. But, for instance, someone convicted of robbery might be allowed to participate if no one was hurt and the inmate had years of good behavior behind bars, while someone convicted of stalking might be excluded even though state law does not define it as a violent crime, Callison said Wednesday.

A statement on the department's website that participating inmates must have no history of violent crimes under California's penal code "was a thoroughly misleading statement," Callison said.

"That was part of the reason I always thought there were no inmates with violent records in the fire camps," he said. "It was inaccurate."

The disclosure shocked officials with the state's firefighting agency and the union that represents professional firefighters who oversee inmates on the fire lines. Union president Mike Lopez called for an investigation after learning of the reversal from the AP on Wednesday.

"I'm very concerned this has been going on without our knowledge and inmates with violent backgrounds have been coming in without our notice," Lopez said.

Janet Upton, a spokeswoman for the California Department of Forestry and Fire Protection, said she also was not aware that unarmed firefighters were supervising inmates with violent histories for years.

"I just don't understand. If that occurred, at some point there had to be some discussion and agreement way back" between the two cooperating agencies, she said.

Tim Williams, a firefighters' union representative who oversees inmates, said CalFire employees don't review the criminal records of the prisoners they supervise.

"I'm going to keep my guard up no matter what, because they're an inmate," he said. "I've got concerns working with inmates every day, but I treat them as a firefighter unless they cause problems, then they become an inmate. They're there for the same function I am, which is to save lives and protect property."

AP's initial story on the nation's oldest and largest inmate firefighting unit drew nationwide attention this week as California endures a deadly fire season amid four years of drought. Corrections officials on Tuesday announced that they were dropping any attempt to add to the number of violent inmates permitted to become firefighters.

But some of those inmates have been serving in the program since at least the 1990s, Callison said Wednesday.

"If the risk of violence is low, then they are minimum custody inmates and they are eligible to serve in the fire camps. As prison officials go, they are considered nonviolent inmates," he said.

As of Sept. 30, he said 1,441 of the 3,732 inmate firefighters — nearly 40 percent — had committed a crime deemed violent under the state's penal code, though all have been classified as minimum-security inmates. He said he is not sure why the department decided to include inmates with violent histories years ago.

The department is considering how to expand the pool of available firefighters as lower-level offenders are being sent to county jails instead of state prison.