Five family members, including a couple and three children, were found slain in their home, and detectives were trying to identify possible suspects, authorities said Tuesday.
Authorities discovered the bodies late Monday afternoon after responding to a 911 call about possible shots fired at the address, Logan County Sheriff Steven Nichols said at a news conference.
A 3-year-old girl survived the attack and was taken to a hospital in Peoria, he said.
Nichols declined to say why his office didn't release information about the slayings until Tuesday morning. He said detectives were following leads to try to identify a suspect, but that authorities could only say that the suspect was considered "armed and dangerous."
CBS News affiliate WCIA in Champaign reported Tuesday that police advised area residents to keep their doors locked.
"We are determined to identify and arrest those responsible," Nichols said.
The slayings occurred at the family's ranch-style home in Beason, a farming community about 140 miles southwest of Chicago.
Nichols would not provide details about the crime scene or the suspected cause of death, but said an autopsy was scheduled for later Tuesday.
Nichols identified the victims and gave approximate ages: Raymond "Rick" Gee,46; Ruth Gee, 39; Justina Constant, 16, Dillen Constant, 14; and Austin Gee, 11.
He did not immediately provide details about the girl who survived.
Others in this tiny community described the family as quiet and well-liked.
Postmaster Jodie Duncan said she saw the children every morning when they came to the post office to get on the buses that took them to school.
"They were great kids," she said. "Austin was in the post office every morning in the lobby on the floor doing his homework. He'd say he's more awake in the morning."
She said Raymond Gee worked in construction and that Ruth Gee was a stay-at-home mother.