The unidentified girl was discovered at an apartment Friday afternoon in cardiac arrest and taken to University Hospital, where she was pronounced dead.
The infant's mother, 19-year-old Sheila Troutman, was questioned but not charged.
Police say Sheppard is Troutman's live-in boyfriend. He was being held without bail.
The girl's 2-year-old sibling was taken into custody by the Division of Youth and Family Services and found to be in good physical condition.
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