At least seven inmates are dead and 17 people are injured after hours-long rioting at a maximum-security prison in South Carolina, according to the state’s corrections authorities.

Several fights broke out among inmates in three housing units at the Lee Correctional Institution about 7:15 p.m. Sunday, and it took authorities more than 7½ hours, until 2:55 a.m. Monday, to secure the prison, officials said.

No officers or staff members were harmed, the corrections department added.

Officials have yet to release the names of those killed and injured. The Lee County coroner told the Associated Press that most of the dead appeared to have been killed by stabbing or slashing.

Emergency crews from at least half a dozen agencies responded to the “mass casualty incident,” according to Lee County Fire and Rescue.

South Carolina authorities helped officials with the Department of Corrections and Lee County secure the prison dormitories where the incidents occurred, said Thom Berry, a spokesman for the State Law Enforcement Division (SLED), who was at the prison until early Monday morning.

Berry said SLED is working with the department “to determine what caused the disturbance.”

Lee Correctional Institution is one of South Carolina’s highest-security prisons, which means the inmates are generally tightly monitored and their movements inside the facility are limited.

Of South Carolina’s nine all-male, maximum-security prisons, Lee Correctional — in Bishopville, about 60 miles northeast of Columbia — is the largest.

The prison houses about 1,600 male inmates, the majority of them in general housing rather than more restricted housing, according to state records.

Violence at Lee Correctional is not uncommon. During the past year, at least three inmates were killed in three separate incidents, while last month, inmates held an officer hostage for about an hour and a half before releasing him, according to the State newspaper.

An investigation by the State’s John Monk found the number of inmates killed across the state’s prisons had quadrupled from 2015 to 2017.

Department of Corrections Director Bryan Stirling told Monk that the trend can be partly attributed to an increase in inmates obtaining cellphones, chronic understaffing, gang rivalries and a higher ratio of violent prisoners to nonviolent ones.

“I’ve been warning about this for a long time,” Stirling told the newspaper, referring to the greater number of contraband cellphones entering the prisons. “These folks are physically incarcerated, but they can continue their criminal ways behind bars. Now, they are fighting over real money.”

On Monday, Gov. Henry McMaster told the AP he had “complete confidence” in Stirling as the head of the Department of Corrections.

According to the department, the number of inmates held in state prisons has declined in recent years after peaking nearly a decade ago. More than 20,400 inmates were held in South Carolina facilities last year, down from a peak of 24,040 in 2010, state records show.

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