Joseph D. Johnson was charged with first-degree murder in the shooting deaths of Joselynn Diaz-Garcia and her 4-year-old son Sunday, police said.
Johnson remains hospitalized under police guard at Loyola Medical Center in Maywood, Joliet police said.
He will be taken to Will County Jail when he is discharged.
Joliet police said 911 calls came in around 6:55 a.m. Sunday, reporting gunfire near Cleveland and Dillman avenues and a shirtless male discharging a firearm near train tracks while running.
Officers then responded to a home in the 700-block of Garnsey Avenue, on Joliet's east side, police said.
There, officers found a 29-year-old man who had suffered an apparent self-inflicted gunshot wound to his ankle, police said.
Outside the home, officers found the 4-year-old boy and his mother, a 36-year-old woman, unresponsive near the backyard. Both victims suffered apparent gunshot wounds and were pronounced dead on the scene.
Officers also found an unharmed 8-year-old boy, who has special needs, in a bedroom. The boy, who is also the 36-year-old woman's child, was taken to a local hospital as a precaution.
A motive for the shooting was not immediately clear, but police said it appears that the person of interest and the victims had some kind of domestic relationship.
Police said the 29-year-old is believed to be a part-time resident of the home, and he is not believed to be the father of the two boys.
"On behalf of the Joliet Police Department, I extend our deepest condolences to the family and to everyone in our community who is touched by this awful and unnecessary act of violence. I am incredibly proud of our officers, detectives, and dispatchers who acted swiftly and professionally under heartbreaking circumstances. We will continue to seek justice while surrounding this family and our city with compassion and support," Joliet Police Chief Bill Evans said.
Joliet police continue to investigate the double fatal shooting.
Anyone with video footage or information related to the incident is encouraged to call the Joliet Police Department Investigation Division at 815-724-3020, or contact Will County Crime Stoppers at 800-323-6734 or online at https://crimestoppersofwillcounty.org/ to remain anonymous.