Calling the incident “tragic” but “very preventable”, the sheriff said a group of family and friends — five adults and the two children — were hanging out inside a Houston apartment on Sunday.
At one point, the children were unsupervised in a bedroom because each parent thought the other was watching them, he said.
“This basically involves a three-year-old and a four-year-old,” Gonzalez said.
“The three-year-old is the alleged shooter. It appears to be unintentional.”
The three-year-old got hold of a loaded semiautomatic pistol, Gonzalez said. The family heard a gunshot and ran into the room, where they found the four-year-old girl unresponsive on the floor.
“It just seems like another tragic story of another child gaining access to a firearm and hurting someone else,” Gonzalez said.
The sheriff said the investigation is ongoing and noted that, in situations like this, it is “very likely” someone faces charges for failing to secure the weapon.
The shooting is just the latest instance of a child using an unsecured firearm and causing tragedy.
About 39 per cent involved a shooter nine years old or younger, the non-profit said.
The American Academy of Paediatrics recommends gun owners keep their firearms locked up and unloaded, with ammo locked in a separate safe.