The shooting happened around 7 p.m. at the Danbury Fair Mall, where investigators say an argument between a group of about eight people people turned violent.
The gunshots sent shoppers scrambling and triggered a lockdown, with some hiding in the back of stores.
Police said the teenage victim was alert and conscious after being struck in the upper chest.
She does not appear to be the intended target, authorities said, though she was a part of the group involved in the dispute.
"It appears that there was another target, and we do have some information that a male was running from the scene carrying a hammer," Danbury Police Chief Patrick Ridenhour said. "We believe he may have been the target, but she got in the way."
She is currently listed in stable condition and is expected to make a full recovery.
No arrests have been made, and the investigation is ongoing.
Anyone with information that can assist with this investigation is urged to contact lead investigator Detective Carroccio at P.Carroccio@Danbury-CT.gov, the Detective Bureau at 203-797-4662, or the Confidential TIPS line at 203-790-8477.
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