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 possible suspects, but that authorities could only say that the suspect was considered "armed and dangerous."
Nichols warned area residents to lock their doors at night as a precaution.
"We are determined to identify and arrest those responsible," he said.
The slayings, which Nichols described as a "brutal homicide against an entire family," 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.
Associated Press writers John O'Connor in Beason and Don Babwin in Chicago contributed to this report.