The decomposing bodies of 28-year-old Latoya Jackson and her three sons, 11-year-old Andrew Simms Jr., 10-year-old Kameron Simms and 5-year-old Kantrell Williams, were discovered in their house in the 8300-block of South Drexel Avenue on July 18, 2015.
Police said they were called to the home after neighbors notice a foul smell coming from the house. An investigator said there were signs of a fire inside the home.
The medical examiner's office released Tuesday the cause of death for each of the older boys. Andrew died of "probable inhalation of products of combustion" in a house fire and Kameron died of "carbon monoxide toxicity" and "inhalation of products of combustion" in a house fire.
Authorities previously released the cause of death of the youngest boy, Kantrell. The 5-year-old died of "probable inhalation of productions of combustion." At the end of July 2015, officials determined the boys' mother was strangled.
All of their deaths were ruled homicides. Police said Tuesday detectives have been "working the case as a homicide" since the medical examiner ruled Jackson's death a homicides.
No one is in custody.
Jackson's relatives said she was raising the boys on her own and had recently moved into the home after losing her grandmother. Friends and family said she was a loving and dedicated mother.
April Bussell, Jackson's best friend, said she was convinced Jackson was in an abusive relationship and that it had something to do with her death.
"She didn't hurt nobody. She wouldn't, I promise you. She would not leave them kids in here. This is domestic violence. This is domestic violence. I had to distance myself because it was that violent. I couldn't watch her do that to herself like that," Bussell said.
The Sun-Times Media Wire contributed to the report.