Updated

The mother of an inmate strangled in a South Carolina prison is questioning how two convicts serving life sentences were able to kill four people without attracting guards' attention.

Becky Kelley of Saint Albans, West Virginia, told The Associated Press on Monday that "the prison guards should've been watching. This should've never happened."

According to arrest warrants, Denver Simmons and Jacob Philip lured each of the four inmates into a cell at Kirkland Correctional Institution in Columbia on Friday morning and worked together to attack and strangle them. Richland County Coroner Gary Watts says the four were dead within 30 minutes.

Kelley's son, 35-year-old Jason Kelley, was expected to be released in 2020.

The mother says she hasn't gotten any answers.

State law enforcement officials declined comment, saying the investigation continues.