The person was not immediately identified, though Guglielmi said he was a juvenile.
Earlier Friday, police superintendent Eddie Johnson said authorities had identified a person believed to have opened fire Thursday into a crowd of young trick-or-treaters, the Chicago Tribune reported. Johnson said the person had been aiming at a 32-year-old gang rival when he shot the 7-year-old girl, who was nearby.
Police believe the gunman is a member of the Gangster Two-Six gang, according to the Tribune. Authorities said he yelled a Latin King gang insult before opening fire around 5:30 p.m. Thursday, the Tribune reported.
Ian Martinez told the Chicago Sun-Times he heard the gunshots Thursday and saw the girl's father scoop her up and take her into a Metro PCS store to wait for paramedics. He told the Sun-Times the girl was very pale and wearing a red or purple bumblebee costume under her coat.
"It's very tragic. We were (expecting) that it was gonna be a fun day," Martinez told the Sun-Times. "Things like this are not allowed. They should do more police inspections around this area."
Authorities told ABC News the girl was shot in the lower neck area and that she was in critical, but stable, condition Friday.
Police continue to investigate.
Original report: According to police, the child was walking on the sidewalk with other trick-or-treaters when someone opened fire across the street, the Chicago Tribune reported.
The girl was shot in the neck and chest and taken to Stroger Hospital. Meanwhile, an unidentified 30-year-old man of no relation to the child was shot in the hand and taken to Mount Sinai Hospital in stable condition, The Associated Press reported.
Lali Lara, who works in a Little Village cellphone store, told the Tribune she heard four shots, went outside and heard the girl's father screaming, "My little girl's been shot."
According to the newspaper, Lara rushed the injured girl and her family into the store and applied pressure to the bleeding chest wound.
"I was pressing on her and calling her name, so she won't close her eyes. She was looking at me, and I was calling her name. She was holding my hand for three minutes, and then she let me go," Lara told the Tribune.
Chicago police spokesman Rocco Alioto called the incident "unacceptable" and told WMAQ, "This was a 7-year-old girl that was trick-or-treating and happened to be shot because a group of guys wanted to shoot another male with a street full of kids trick-or-treating."
Police do not have descriptions of the suspects, who were chasing another man when they opened fire, and no weapon has been recovered, the station reported.
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- The Associated Press contributed to this report.