Authorities arrested Stephen Procopio, 19, of New Wilmington, on Wednesday night and charged him with three counts of homicide.
"I didn't hurt her," Procopio told WPXI-TV in response to a question about the girl's death as he was taken into custody.
A relative found the bodies of 31-year-old Nichole Pumphrey, 10-year-old Amariah Emery and 31-year-old Lawrence Cannon in a New Castle home on Tuesday. Officials said the woman was shot twice and the other victims were each shot once.
Police Chief Robert Salem told WFMJ-TV Procopio knew Pumphrey. "We believe he used to babysit the children. He was an acquaintance with Nichole," the chief said.
KDKA-TV reported Procopio and a woman were taken into custody for questioning and later released on Tuesday. He was again taken in on Wednesday night.
Four other children were found in the home. All were interviewed and then released to relatives.
Police did not disclose a motive.
Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.