The babysitter told state police she put the girls down for a nap Wednesday and then took a nap herself. When she awoke, 20-month-old Jenna Walker and 2-year-old Maggie Kovski were gone, state police Cpl. Mark Zaleski said.
Rescuers were called at about 11:15 a.m. after the baby sitter found the bodies floating in a small, man-made fishing pond about 100 yards (90 meters) away from the home. Chris Barber, chief of Stancliff Hose Co., said the 18-year-old sitter was frantically trying to help the children when emergency workers arrived.
Authorities did not identify the sitter, who is a relative of Jenna and lived in the house in this rural Pennsylvania town,15 miles (24 kilometers) south of Erie. Maggie was a neighbor.
Investigators said Thursday afternoon that they believe they know how the toddlers got out of the house and to the pond, but would not disclose details.
"I'm sure that everyone understands that this is just a horrific tragedy for everyone involved," Foulk said.
Coroner Lyell Cook determined that the girls drowned and ruled the deaths accidental.
The ruling would not preclude criminal charges if investigators determined the baby sitter was negligent or reckless, Cook said.