It happened just before noon in a second-floor bedroom of a home on the 1600 block of North 29th Street, the department said.
The girl was taken to Temple University Hospital by private car where she was pronounced dead at 12:33 p.m., according to authorities.
The suspect, a 14-year-old boy, lives in the home but it was unclear to police Thursday afternoon if he was related to the victim. The weapon has been recovered, they added.
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