In a tragic end to an emotional search for a pair of missing toddlers, the bodies of the tiny brother and sister turned up Monday in an algae-covered sewage pond near their home, FOX News has confirmed.

Jonah and Nicole Payne, 3 and 2, were found dead about 12:15 p.m. Monday just a few hundred yards from their lakeside house, according to Georgia Bureau of Investigation (search) spokesman John Bankhead.

"They were positively identified by the father," Bankhead told reporters in a brief news conference Monday afternoon. "This has turned into a death investigation from a missing persons investigation. There's not much more I can say."

It was not immediately known whether the youngsters were the victims of foul play or just an accidental drowning, according to Bankhead, who said autopsies were planned for Tuesday morning.

A road from the house the children lived in with their parents leads directly to the 1 1/2-acre, bacteria-filled sanitation pond where their bodies were discovered, he added.

Green algae had made it difficult to see the children when authorities first searched the pond over the weekend, Bankhead said, but a second look Monday revealed a bulge in the algae that turned out to be one of the toddler's bodies. The other was found nearby.

The pond — part of the city's sanitation system — is surrounded by a fence. But the fence has several holes where a child could easily get through.

Authorities said Nicole needed a breathing machine daily and her brother was on medication for an unspecified condition.

The children were reported missing Saturday evening after last being seen at their Warren County (search) home, in east Georgia about 100 miles from Atlanta.

The mother had told police she was in another room Saturday and emerged to find the front door open and the tiny siblings missing. She reported their disappearance to authorities early Saturday evening.

Earlier the same day, the little girl had managed to unlock the locked door and escape with her brother about 4 p.m. but a neighbor found the toddlers and they were returned a half hour later, according to Police Chief Jim McClain. Nicole had figured out how to unlock the front door and would do it when no one was around, he said.

The exact circumstances of the disappearance of the little girl and her brother were not known. It was "way too early" to determine how long the children had been dead, said Bankhead.

Before the tragic discovery, polygraphs were being used in the investigation, according to Bankhead — who did not elaborate on whether the parents were suspects or who was given a lie detector test.

"There have been more than one polygraph administered," Bankhead said before the children's bodies were found.

The mother, Lottie Kain, reportedly took a polygraph test Sunday but the results have not been released. Kain is engaged to Dennis Payne, the father of the children.

Kain was also questioned by authorities Sunday evening and the search was temporarily halted at that point.

The family does not have a history of abuse involving the children.

Bankhead also said Monday that authorities had already checked into the possibility that a sex offender in the neighborhood had abducted and killed the toddlers. There have been several recent abductions and murders of children in Florida, Georgia and other states at the hands of convicted sex offenders living in the area.

"They ruled that out very quickly," Bankhead told reporters. "That's one of the first things they've checked."

The children's disappearance shocked and saddened people in Warrenton (search), a town of narrow, two-lane roads known mostly for the sweet potatoes at Jane's Restaurant.

Stephanie Johnson, walking with her 22-month-old daughter, Ella, said she had been praying for the family.

"There's a part of me that wants to stay inside all day, but then I remember that this kind of thing doesn't happen in a small town like this," Johnson said.

FOX News' Jonathan Serrie, Catherine Donaldson-Evans and The Associated Press contributed to this report.