Police said Santos Diaz, 45, was arrested Thursday afternoon.
Diaz allegedly fired shots outside a home on the corner of East Ontario and Emerald streets, in the city's Kensington section, around 10:40 a.m. Sunday.
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The baby was being held by her mother when one of the bullets struck her hand.
The girl was taken to St. Christopher's Hospital for Children where she was placed in stable condition.
The baby's mother was not injured.
Police say a 17-year-old suffered a graze wound to the leg but he did not require any medical attention.
"The individual was part of the gathering," Lt. Robert Brockenbrough with the Philadelphia Police Department said on Sunday. "There's been a lot of gun violence in Philadelphia, so you just have to be careful. This started as a friendly gathering and escalated to this."
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