A three-year-old boy shot his 18-month-old brother dead in Arizona on Wednesday, according to police officials in the US state.
The boys and their mother were visiting a neighbour when the siblings found a semi-automatic handgun and took it to another room at the property in Payson, a town around 90 miles (145km) north-east of Phoenix.
Neither adult knew that the young children had taken the weapon, according to Payson Police Chief Don Engler.
Officers are currently investigating the shooting – a procedure which will take around a week, Mr Engler said.
Mr Engler said the probe will focus on how securely the gun was kept in the apartment, and added the weapon was not in plain sight in the living area of the neighbour's apartment.
Gila County Attorney's Office will be given the resulting documents, in order to decide whether anyone will be prosecuted in connection with the baby’s death.
The shooting comes days after Elliot Rodger shot and killed six people, before turning the gun on himself, in the US state of California.