Story highlights Jeremy Roberts, 38, will be charged with murder in his ex-wife's death, police say

"He just ran outside and killed my mom," Roberts' 13-year-old son told a 911 dispatcher

Roberts got on the phone, calmly telling police he had stabbed his ex-wife

The son who called 911 is haunted by what he saw, his older sister says

Barely holding back his sobs, a 13-year-old boy makes a heart-rending 911 call to the Grove City Police Department in Ohio.

"My dad just killed my mom. He just told me to call you guys," the boy is heard telling a 911 dispatcher. "He just ran outside and killed my mom. And she's laying in the grass in front of the apartment."

The boy's father, Jeremy Roberts, had the boy make the call, police said, after allegedly stabbing his ex-wife to death outside his apartment.

Roberts then calmly got on the phone himself, giving the apartment's address and informing police that the "only" weapon -- a knife -- was outside with his ex-wife, Candice Roberts.

"What did you do?" the dispatcher asks him.

"She's covered in a blanket. You guys will see," Roberts responds in an even tone.

He can then be heard, on the nearly five-minute 911 call, telling his children that they have a "good big sister (who) will take care of you guys."

"I told you that there's a lot of things that you don't understand, and you don't know. And now there's no choice for them to come out," Roberts said in the recorded call, without detailing what he meant.

Before going to the front door -- and handing the phone back to his 13-year-old son -- Roberts says, "One more hug. I love you guys." A small child can be heard crying in the background.

The 13-year-old boy can be overheard telling his father, "I don't want to see you right now."

Roberts, 38, is in custody and will be charged with murder in connection with his ex-wife's death, police said.

The victim had gone to the suspect's apartment in Grove City, about 10 miles southwest of Columbus, before 9 a.m. Sunday to pick up their two children, police said.

Afterward, the 13-year-old boy phoned police to tell them his mother was dead and his father was responsible.

Candice Roberts' 16-year-old daughter told CNN affiliate WCMH she feels "incomplete and abandoned" after the death of her mother, who she said "was always there for me."

Her brother is haunted by what he saw, she said.

"My brother says that every time he opens his eyes, he sees my mother's face and her yelling, 'Help me!' " the teenager said. "And he sees my dad standing there with the blood all over him, and the knife in his hand."

The daughter added that she "would do everything for him to be held accountable."

"You don't just get to murder somebody and get away with it," she said.