Recovery crews in southern New Mexico were working to reach a child's body at the bottom of a 30-foot-deep hole Sunday.
Authorities believe the child is Samuel Jones, who went missing Saturday, Carlsbad Police Department spokeswoman Lt. Jennifer Moyers said in a statement.
Officials think the child died when he fell into the hole in his neighbor's yard, CNN affiliate KOAT reported.
"We initially began with rescue procedures. Unfortunately, it was discovered that the conditions inside the hole were not survivable, the amount of oxygen that was there, the temperature, that type of thing," Carlsbad Police Chief Daniel Fierro told CNN affiliate KRQE.
A detective searching for the 4-year-old Sunday morning spotted the body when he shined a light into the neighbor's well, KOAT reported.
It could take a day for mining crews to reach the body safely, state police said.
"It breaks my heart, that little boy down there," heavy equipment operator Paul Kartscner told KOAT.
Crews were digging a trench from the side of the hole and shoring up the sides, KOAT said.
Investigators, who said Saturday that they feared the boy had been abducted by a stranger, said Sunday that they were treating the case as a homicide until they determine a cause of death, KRQE reported.