0812 GMT May 21, 2018
Fights among inmates erupted Sunday evening at the Lee Correctional Institution in Bishopville, South Carolina and was brought under control at 2:55 a.m. on Monday, AFP reported.
"The incident at Lee CI resulted in 17 inmates requiring outside medical attention and seven inmates were killed," the department said on its Twitter account.
All prison guards and law enforcement authorities who responded were "safe and accounted for," it said.
The department had initially described the riot as an "ongoing incident," only revealing the scope of the problem hours after its start at 7:15 p.m. Sunday.
"The incident involved multiple inmate on inmate altercations in three housing units," the department said.
Lee Correctional Institution, which is reported to have capacity to hold nearly 1,800 inmates, is a maximum security facility built in 1993 to house violent offenders and inmates who exhibit behavioral problems.
Deadly prison riots have been relatively rare in the United States, despite some notable exceptions.
In 1993, nine inmates and a corrections officer were killed in the Southern Ohio Correctional Facility, a maximum security prison in Lucasville, Ohio.
A two-day riot and hostage-taking at the New Mexico State Prison in 1980 left 33 inmates dead and 200 injured, one of the worst in modern US history.
Probably the most famous was the 1971 rebellion at New York's Attica prison.
After a four day standoff with inmates holding 42 hostages, New York state police moved in to retake the prison on orders of governor Nelson Rockefeller. By the time it was over 43 people were dead, including 10 guards and prison employees and 33 inmates.