WATERFORD, Pennsylvania (AP) -- Two toddlers who wandered away from home while their teenage baby sitter slept accidentally drowned in a nearby pond in northwestern Pennsylvania, officials said.

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 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.

"She was performing CPR, and when I got there, I started CPR on the youngest one," Barber said. "They had no pulse. They were not breathing."

Authorities did not identify the 18-year-old sitter, who is a relative of Jenna and lived in the house in this rural Pennsylvania town,15 miles south of Erie. Maggie was a neighbor.

Investigators said Thursday afternoon that they believe they know how the toddlers got out of the house, got to the pond and other details. District Attorney Brad Foulk said he would not discuss those details to protect the family's privacy and because authorities planned to conduct more interviews.

"I'm sure that everyone understands that his is just a horrific tragedy for everyone involved," Foulk said.

A woman who answered the phone at the Walker home said the family did not want to talk about the accident. The Associated Press could not locate a telephone number for Kovski's family.

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.