SUMTER, S.C. — A former soldier shot and killed three children as they slept in their South Carolina home Tuesday night while their mother frantically sought help, authorities said. The ex-soldier then killed himself.

Charles Slacks Jr. also killed an Army soldier who worked with the children’s mother and happened to be at the home, Sumter Police Chief Russell Roark said at a news conference Wednesday.

Slacks and the woman were divorced, but he still had a key to the home in Sumter and let himself in around 10 p.m. Tuesday, Roark said.

Slacks shot the co-worker in the backyard as the mother tried to stop him, pushed past her to go upstairs, and shot the children, ages 5, 6, and 11, in their beds, the police chief said.

“Little children were sleeping in the comfort of their own bed,” Roark said.

The mother’s cell phone was upstairs charging, so she ran to see if her wounded co-worker had a phone she could use. She heard the gunshots, came back inside and saw her ex-husband at the top of the stairs, where he shot himself, Roark said.

The soldier who was killed had no relationship with the woman outside of work, and investigators haven’t figured out why Slacks came to the home or whether he was supposed to have a key, Roark said.

“Anybody who has children — anybody who has empathy toward a child dying so violently could understand how that mother is feeling,” the police chief said.

Slacks, 42, killed his own children, 5-year-old Aayden Holliday-Slacks and his 6-year-old brother Aason Holliday-Slacks, and also killed their half-sister, 11-year-old Ava Holliday, Sumter County Coroner Robbie Baker said. Before the divorce, Slacks had been Ava’s stepfather.

“It was a long night. This was just an awful thing,” the coroner said.

The identity of the soldier killed has not been released because officials are still trying to determine his next of kin, Baker said.

Slacks was a civilian assigned to U.S. Army Central at Shaw Air Force Base, while the soldier killed was also assigned there, according to information from the Army.

The Sumter School District is grieving the loss of three students, including two siblings who attended Millwood Elementary School and a third who attended Alice Drive Middle School, Superintendent William Wright said in a statement.

State Sen. Thomas McElveen, a Democrat who represents Sumter, is a young father of three himself and led the state Senate in a moment of silence.

“As a father, I am absolutely nauseated up here talking about it,” McElveen said.

(WATCH: Ex-army soldier admits to stabbing fellow soldier for reporting marijuana use)

Ex-army soldier admits to stabbing fellow soldier for reporting marijuana use Hawk’s body was found with 40 separate stab or slash wounds in his Fort Stewart barracks room the next day. (NCD)

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