Police said a 63-year-old man and a 48-year-old woman were found unresponsive in the Garfield Ridge neighborhood's 5100-block of South Lotus Avenue around 3:40 p.m.
Video from the scene shows a large police presence at Mark Twain Elementary School. A stretcher could be seen, and tape blocked off part of the school's parking lot.
Police said each adult suffered a single gunshot wound to their head. Both were taken in critical condition to Christ Hospital, where they were pronounced dead.
What led up to the shooting was not immediately clear, but CPD is only classifying one of the deaths as a homicide. Investigators recovered a weapon from the scene.
"The only facts we know right now are that two families have lost a loved one and our prayers are with their families, and the staff, students, and families at Mark Twain Elementary School," said SEIU Local 73 Executive Vice President Trumaine Reeves.
Local 73 represents custodial staff at CPS.
Chicago Public Schools is not currently in session. Class starts in two weeks.
The school's principal sent a letter to parents and guardians, saying, "It is with deep sadness that I inform you that this afternoon, shots were fired inside our school building. I am sorry to share that members of our school community were impacted. We immediately contacted the Chicago Police Department (CPD) and the CPS Office of Safety and Security for support. No students were present in the school building during the time of the incident. For privacy reasons and due to the active investigation, I cannot share additional details at this time. We know that this news will be deeply concerning to our families and staff. We will work to provide additional information to the school community as the investigation proceeds. Please know that we are taking this situation very seriously to ensure the safety of our school community. We thank you for your continued partnership."
Area One detectives are investigating.
This is a developing story. Check back with ABC7 for updates.