The children ranged in age from 3 to 7.
The victims' bodies were found Monday by an officer who went to the apartment in this central Oklahoma community to check on the welfare of the occupants, said Oklahoma State Bureau of Investigation spokeswoman Jessica Brown.
Joshua Steven Durcho of El Reno was being sought as a suspect in the case, Brown said. El Reno is abough 25 miles west of Oklahoma City.
State Medical Examiner's office spokeswoman Cherokee Ballard said the deaths were homicides but declined to elaborate.
The victims were identified as Summer Garas, 25; daughters Kirsten Rust, 7; Autumn Rust, 6; and Evynn Garas, 3; and son Teagin Rust, 4. Court records show that Summer Garas and her husband, Jason Garas, were divorced on Dec. 10.
Durcho, 25, was Summer's boyfriend, said James Franklin, whose wife is Summer Garas' former mother-in-law.
"That was wrong, there is nothing that can justify killing a mother and four kids," Franklin said. "I have to deal with this the rest of my life. I can't see my grandkids."
Prison records show Durcho was convicted of car theft as a juvenile and was sentenced to four years in prison in 2003 for unlawful possession of marijuana. He was released in February 2007, records show.
Brandy Higgins, whose mother lives in the same apartment building, said her mother heard a loud noise before police arrived about 4:00 Monday afternoon.
"All I know, my mom heard a loud boom, and I get a phone call," Higgins said.
She said her mother, Janice Dykes, 56, is disabled, spends much of her time around the apartment and had become close to the children who were slain.
"She's grown attached to the kids," said Higgins, 35, of El Reno. "Those were grandma's babies."
Higgins' own children also played with the youngsters. Higgins said she hopes whoever did the crime suffers for his actions.
"I hope it eats him alive," she said. "Them babies won't experience life."