Shoddy electrical work aboard a houseboat floating beside the boys was the likely culprit for the deaths of 10-year-old Noah Winstead and his friend, Nate Lynam, 11, the Knoxville (Tenn.) News Sentinel reported.
"We did find some frayed electrical wiring under some steps on the boat," Grainger County Sheriff Scott Layel said. "That could be a factor, and that's obviously something that will be looked at during the investigation."
The houseboat was registered to Nate Lynam's grandfather, Michael Voccola, officials said.
On Wednesday, the two boys were swimming next to the boat in Cherokee Lake.
"The children were swimming and began screaming," Layel said. "At first everyone thought maybe they were being stung or snake-bit. Everyone began to jump in and attempt to save them."
Five adults who jumped in the water to help the boys suffered shocks as well, Layel said.
"One gentleman said he was so paralyzed [by the electrical current] he went under maybe 3 or 4 feet," the sheriff said.
The adults and an 8-year-old girl were treated at a local hospital for minor injures caused by the shock, officials said.
Winstead died at the scene and Lynam died Thursday night at a hospital in Knoxville, authorities said.
An electrical investigator from the state fire marshal's office in Nashville inspected the houseboat Thursday, officials said.