COLUMBIA, S.C. (AP) — Two convicts each serving a life sentence for killing a mother and her child have been charged with murder in the strangulation deaths of four inmates inside a South Carolina prison.

Denver Simmons and Jacob Philip lured each of the four inmates into a cell at Kirkland Correctional Institution in Columbia on Friday morning, then worked together to attack and choke them, according to arrest warrants released early Saturday.

Two of the inmates were also beaten or stabbed with a broken broomstick, according to the warrants released by State Law Enforcement Division agents.

The attacks took place within 30 minutes, Richland County Coroner Gary Watts said in a statement Saturday.

One inmate was choked with an electrical cord, while another had his chest pressed down while he was being strangled. The cause of death for the other two inmates was also strangulation, Watts said.

The warrants gave no reason for the attacks. They did say there is video of the killings and the men confessed to investigators.

The Corrections Department will conduct an internal investigation into the killings after the criminal investigation is finished, Corrections Department Director Bryan Stirling told The Associated Press on Saturday.

Because of the ongoing investigation, Stirling would not talk about whether prison policies were broken or how the inmates came to be together.

Prison records show the four inmates killed were considered minimum security risks. The Kirkland prison serves several roles in South Carolina's system. It operates a specialized housing unit for the state's most dangerous inmates, an assessment and evaluation center for new inmates sentenced to more than three months, and a 24-bed infirmary, according to the Corrections website.

Simmons, 35, has been behind bars since his 2007 arrest on charges he killed a woman he knew in Colleton County, took her debit card and ate pizza for lunch, then got her 13-year-old son from school and killed him too.

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Philip, 25, has been in prison since 2013. Authorities said he strangled his girlfriend and her 8-year-old daughter in Berkeley County.

Prison records show Simmons has three disciplinary infractions in four years for being out of place, disrespect and refusing to obey an order. Philip has no disciplinary actions against him in nearly two years in state prison.

Records did not show if the inmates had lawyers.

The four inmates killed were John King, 52; Jason Kelley, 35; Jimmy Ham, 56; and William Scruggs, 44, prison officials said.

King was serving time for a variety of crimes and had a projected release date of October 2020. Kelley was serving 15 years for assault and battery of a high and aggravated nature. He was scheduled for release in August 2020.

Ham was scheduled for release this November after serving a sentence for a variety of offenses. Scruggs was sentenced to life in prison for murder and first-degree burglary.

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Associated Press writer Meg Kinnard contributed to this report.