An autopsy was planned by the Georgia Bureau of Investigation, Muscogee County coroner Bill Thrower said.
Investigators have said they don't know what led to 28-year-old Eddie Harrington's decision to leave his Columbus home with his twin 23-month-old girls and his 3-year-old son. He reportedly lived with his girlfriend and the children.
At some point, Harrington then sent the letter to his father in the Indianapolis area "indicating he was going to kill the kids," FBI spokesman Stephen Emmett has said.
Columbus police issued a felony child endangerment warrant for Harrington. A spokesman did not return several phone calls seeking comment on Wednesday.
Federal authorities thought he had left the state and issued a warrant charging him with unlawful flight to avoid prosecution. It was not clear whether Harrington ever did.
The four had been missing since at least March 6, when authorities in Tennessee issued a child abduction alert after Harrington may have been spotted there.
Nadia Hudson, who lives across the street from Harrington, said she thought it was strange that she hadn't seen Harrington's green car in a while.
"I remember his kids, because I remember when I would bring my little boy outside to play, he would holler over to the twins," Hudson said.
Hudson, who moved into her home four months ago, said she had never spoken to Harrington.