Putnam County Coroner Dr. Hari Chakravorty said an autopsy revealed that Pamela Kaner, a friend of the girl's family, did not drown. The 59-year-old woman suffered from hypertensive heart disease and "did suffer a heart attack," he said.
The finding is preliminary until toxicology tests are completed, he said.
The girl, whose name has not been made public, was rescued Monday when boaters heard her crying on Lake Gleneida in Carmel. They found her holding onto the body, loaded her into their boat and took her to shore.
Carmel Police Chief Michael Johnson said the girl told police that Kaner, of Brewster, who was caring for her while her mother ran an errand, took her into the water and was holding her when something went wrong.
The bodies of drowning victims usually sink, then return to the surface as they decompose, Johnson said Tuesday. Kaner's body was retrieved by firefighters, who paddled out in a commandeered boat.
The girl was treated at a hospital but was not seriously injured.
The lake, about 730 yards at its widest, is part of New York City's water supply system, and swimming there is banned. DEP spokesman Ted Timbers said no drinking water has been drawn from the lake for more than a year because it's part of a section that has been offline while a filtration plant is built in the Bronx.
The shore of the lake, which abuts the main road of the hamlet of Carmel, is littered with rowboats, most chained or cabled to trees. Johnson said the DEP grants permits for the boats. Signs on the shore say, "Recreation by permit. Entry for other purposes prohibited."
"She shouldn't have been in the lake," Johnson said.
Only ducks and gulls were on the water on Tuesday afternoon.
Kerry Browne of Carmel, a house renovator, said, "On a nice day like this, anybody would like to jump in the lake, but you know the rules." He said he hoped the girl would be able to recover from "holding onto a body like that."