An autopsy was scheduled for Tuesday on Parwin Husan, who appeared to have been tied to a radiator and somehow broke free, authorities said. She then either jumped or fell from the window.
Neighbors said they cared for Husan for five hours after she fell last month until her mother returned. Some neighbors then spoke to the girl's stepfather.
"We told him, 'Don't leave this girl inside the apartment because something is going to happen to her,'" said neighbor Shamel Al-Dhahri. "He got mad, so we didn't talk about it" again.
Police have questioned the mother and her boyfriend. No charges had been filed, Chicago Police Department spokeswoman Laura Kubiak said.
"It's a pretty tragic case," said police spokesman Pat Camden. "We need to figure out whether this was an accident."
Officers were called to the scene after a neighbor saw Husan lying on the pavement behind the apartment building Sunday evening. Neighbor Hussam Al-Shimari said the girl was moaning and barely conscious.
Police said she appeared to have been left alone in the apartment.
The mother has not been identified.
"This is being investigated as a child endangerment case," officer Matthew Jackson said.
The Department of Children and Family Services has taken Husan's 3-year-old brother and 5-year-old sister into custody and placed them in a foster home.
The family had no prior contact with the agency, spokeswoman Martha Allen said.
"It's not the first time they left her alone," Al-Shimari said. "I don't know how come they left her alone. You can't leave a dog alone for many hours ... This is a kid, you've got to think."