LINCOLN, Neb. — Nebraska prison officials have been trying to overhaul an understaffed, overcrowded prison system for years, but they say violent outbursts by inmates keep hindering their efforts.

In the last two years, prisoners have started a deadly riot, escaped from a maximum-medium security facility and assaulted dozens of staff members. Last month, inmates sent nine employees to the hospital and triggered a facility-wide lockdown at the Lincoln Correctional Center.

Although some violence is a routine part of prison life, administrators say the repeated incidents require a large amount of time, money and manpower that could otherwise be spent on improvements.

“It’s really frustrating,” Department of Correctional Services Director Scott Frakes said Wednesday after a legislative hearing. “I told the population when I arrived that we’re going to change things. We want to make it better. But these acts of violence continue to consume a disproportionate amount of resources.”

Frakes said last month’s assault on Lincoln Correctional Center staff members required a response from hundreds of employees.

“Some of them would have been doing the same things they’re doing today — supervising inmates — but many of us would have been focused on moving initiatives forward,” he said.

On Friday, the department announced that two housing units at the Tecumseh State Correctional Institution were going on lockdown status and two other units would have restrictions placed on them in response to a security threat. Administrators said they had received word of “serious violent attacks” planned against staff members.

The May 2015 riot, escape of two sex offenders in June and assault on nine employees last month all led to prison lockdowns, which are even more taxing on staff members.

When inmates are restricted to their cells, most of their routine jobs fall to prison employees. Behavioral health specialists sometimes find themselves preparing three meals a day for hundreds of inmates and delivering them to cells one at a time, said Doug Koebernick, inspector general of the Nebraska Correctional System.

Inmates who are confined to their cells are also unable to access treatment and programming to help them deal with their problems, Koebernick said.

“You have guys who are angry and violent, and you’re not doing anything with them,” Koebernick said. “It’s not like they’re going to change while sitting in their cell.”

Sen. Les Seiler, the chairman of a prisons oversight committee, said staffing shortages and inmate overcrowding are to blame for many of the recent problems.

“When you’re understaffed the way they are … it shows a weakness,” said Seiler, of Hastings. “Prisoners are taking advantage of that weakness. I don’t think there’s any doubt that staffing and overcrowding are the root causes.”

Seiler said state officials need to increase pay and improve working conditions for prison staffers to reduce high turnover and burnout rates among employees, which creates a larger workload for those who remain.

State officials on Friday proposed a series of pay raises for corrections employees, ranging from 2.4 percent for mental health professionals to 11.8 percent for prison sergeants.

The proposal “is a significant investment in our corrections workforce, and a serious step toward addressing staff and retention,” Gov. Pete Ricketts said in a statement.

Seiler said legislative reforms are slowly starting to reduce the prison population by ensuring that more inmates have access to treatment and rehabilitation programs. The programs have been shown to reduce recidivism, a major contributor to the overcrowding.