A 15-year-old girl and her 16-year-old boyfriend are accused of shooting and killing her family members.
Two other family members survived.
The husband of a survivor revealed what he says was the motive.
"There was a list on Instagram that they, her and the boyfriend made. So we were aware, we just didn't know when," Lamarion Smiley said.
"They took them to Jones Park and they fired off in the vehicle. Hitting my wife four times, killing Quentin and Shania Thompson and shooting my auntie in the face," Smiley said.
On Tuesday prosecutors charged 16-year-old Ja'ymier Davis with 12 felonies including the attempted murder of his girlfriend's mother and cousin: Tiffany Thompson and Santosha Scott.
"She stood over my wife trying to kill her, but they had no more bullets," Smiley said.
Smiley said he believed their relationship was the motive.
"It's all because y'all wanted to be together, so she wanted, they wanted to eliminate everybody that had a problem with the situation," he said.
The family says the gun used in the shootings was stolen from the 15-year-old's mother before the shooting.
"[She] took the keys from her purse, unlocked the lockbox and took the firearm," Smiley said. "The police were notified as soon as they found out that the firearm was taken."
Charges of using the stolen weapon were also filed in court along with dismembering a human body, after investigators say the grandmother's thumb was cut off.
Prosecutors are recommending that the 15-year-old girl be tried as an adult.
ISP said the investigation began when human remains were found at the Samuel Gompers Homes on Sunday. The victim was later identified as 49-year-old Cherie L. May of East St. Louis.
An investigation led to the identification of four other shooting victims at multiple locations in the city: 25-year-old Shania W. Thompson, 24-year-old Devin May, 21-year-old Quentin L. Thompson, and 74-year-old Patricia A. May.