The fire early Sunday may have stemmed from a family dispute, a relative said. The children's parents, who tried in vain to save them, were among the injured, the relative said.
Zachary Q. Meeks, 27, was arrested Sunday at his home and is facing preliminary charges of first-degree murder, aggravated arson and arson, police said. A prosecutor said formal charges would be filed Monday.
Meeks has several previous felony drug arrests, including a 2005 conviction that landed him a four-year prison sentence, according to the Illinois Department of Corrections. Department records show Meeks was released from prison in December and was to be on parole for a year.
Rescuers responded to a house engulfed in flames at about 3 a.m. and later discovered the five bodies inside. Neighbors said flames were shooting out the windows of the two-story brick home, and firefighters said they could see smoke from at least a quarter-mile away.
The four boys and one girl were found on the second floor of the house, coroner Gary Hamilton said.
He identified the children as Kendall Edwards, 10; Althea Clark, 9; Camron Clark, 5; Khalil Clark, 3, and Kejuan Clark, 5 months.
Hamilton declined to release preliminary causes of death and said autopsies are planned.
One person was airlifted to a Springfield hospital in critical condition, said Police Sgt. Doug Schlueter. A firefighter suffered minor burns to his face, and two other people were treated at a hospital and released, authorities said.
Quincy Fire Chief Scott Walker told CBS affiliate KHQA-TV, "There was a woman injured in the fire who attempted to make entry. She attempted to crawl into a window and sustained burns. She managed to push past firefighters and apparently made an attempt in a sense of panic."
The children's aunt, Michelle Gavin, told WGEM-TV in Quincy that Meeks, who is her cousin, poured gas on the front of her sister's house and set it on fire.
The parents, Jeanette and Keith Clark, were down the street when the fire started, Gavin said. She said Keith Clark was the person who needed to be airlifted to the Springfield hospital.
Gavin said there was a dispute between the victims' parents and Meeks stemming from a drug-related prison sentence handed down to another family member.
Neighbors said the children were well-behaved.
"They were well-dressed and clean, and they always said hello," Sue Johnson said.
"I was horrified because I can still visualize those kids playing in that yard and in the street. They were well-behaved, nice kids," Johnson told KHQA.
State and local authorities were investigating the fire. Quincy, which sits along the Mississippi River, is about 90 miles west of Springfield.