Daron Mayes was shot in the back of the head around 8:20 p.m. Sunday in his Paterson home.
The 6-year-old, who police declined to identify, was playing with Mayes when the pair found the gun, said Chief Assistant Passaic County Prosecutor John Latoracca. Authorities said the two were related but declined to specify.
"In situations where the child who is responsible - quote-unquote - is also that young, just understanding the trauma that that child is going through and is going to continue to go through, potentially for the rest of his or her life, it's really a horrible, horrible thing," Latoracca said.
Authorities were looking for a 23-year-old relative who lives in the home and owns the gun. Police believe the weapon might not be registered and may bring charges against the gun owner, Latoracca said.
The prosecutor declined to name the relative.
"The responsible adults here certainly had no reason to believe there was a loaded handgun in that bedroom," Latoracca said. "They are just devastated, as you can imagine."
