After a string of inmate deaths in Mississippi that began late last year, the Justice Department announced Wednesday that it is opening a civil rights probe into the state's penitentiary system.

The department's civil rights division says it will examine conditions at four Mississippi prisons, including the state penitentiary at Parchman, the state's oldest, where a prison riot broke out on Dec. 29 after an inmate was killed. The all-male prison includes the state's death row.

Since December's riot, a total of 15 inmates have died. Officials say two of the deaths were apparent suicides by hanging, but that many of the others are thought to be gang-related killings. The Mississippi Department of Corrections says 29 staff members have also been assaulted over the same period.

"The investigation will focus on whether the Mississippi Department of Corrections adequately protects prisoners from physical harm at the hands of other prisoners at the four prisons, as well as whether there is adequate suicide prevention, including adequate mental health care and appropriate use of isolation, at Parchman," a DOJ statement said.

The Justice Department says its investigation will focus on conditions at the Mississippi State Penitentiary, the South Mississippi Correctional Institution, the Central Mississippi Correctional Facility and the Wilkinson County Correctional Facility, which is run by a private contractor paid by the state.

The violence in Mississippi prisons has drawn national attention, particularly because of the advocacy of entertainment figures such as hip-hop artist Mysonne and Jay-Z, whose company, Roc Nation is funding a lawsuit against the state demanding improved conditions.

In a class-action lawsuit on behalf of prisoners at the East Mississippi Correctional Facility filed in 2013, the American Civil Liberties Union, Southern Poverty Law Center and others charged that "the Mississippi Department of Corrections has operated the prison in a continuous state of crisis, neglect, and abuse for years, causing extreme and preventable suffering for thousands of prisoners in violation of the Eighth Amendment's prohibition on cruel and unusual punishment."

Mississippi has one of the highest incarceration rates of any state. Long hours and low pay for prison guards has meant that many positions go unfilled. According to a report by ProPublica, rather than counting inmates, as required, some guards falsify their counts.

"The state has sharply cut its spending on prisons over the last few years," the ProPublica report published in August says. "Along the way, the number of guards at the three state-run prisons has plummeted, from 905 in July 2017 to 627 two years later, even as the number of inmates has remained the same. Vacancies abound, largely because the pay is so low."

Jimmy Anthony of the Mississippi Association of Gang Investigators told state lawmakers this week that gangs such as the Gangster Disciples, City Royals and Vice Lords are operating in the prisons and engaged in narcotics and other criminal activity.

"It's not popular to admit [that] you have a gang problem," Anthony said, according to Mississippi Public Broadcasting. "Nobody wants to hear it. It scares people."

The Justice Department's use of the Civil Rights Division to investigate state law enforcement practices, not uncommon during the Obama administration, has become rare under President Trump.

However, a spokesperson for Gov. Tate Reeves welcomed the move.

"We are grateful that President Trump's administration has taken a focused interest in criminal justice reform and that they care enough about Mississippi to engage on this critical issue," the governor's spokesperson, Renae Eze, said. "As we continue our own investigations, we look forward to cooperating with them and working together to right this ship."

NPR's Debbie Elliott contributed to this report.

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