Payson Police Chief Don Engler says his department's investigation of the Tuesday shooting will take about a week. He says results will be forwarded to the Gila County Attorney's Office for a decision on whether to prosecute anybody.
Engler says the probe centers on the securing of the gun, which he says was not in plain sight in the living area of the neighbor's apartment.
The boys and their mother were visiting the neighbor when the boys found the semi-automatic handgun and took it to another room. Engler says neither adult knew the boys had the weapon.
Payson is about 90 miles northeast of Phoenix.
Names of those involved weren't released.