A 3-year-old girl in Houston gained access to a firearm and "unintentionally" shot and killed her 4-year-old sister late Sunday, Harris County Sheriff Ed Gonzalez said.
Gonzalez said the preliminary investigation showed that the two children had been inside an apartment with five family members and friends and each of the two children's parents believed the other was watching the kids. Instead, they were left unsupervised and picked up an "unsecured" and loaded semi-automatic pistol.
The family members heard one gunshot and ran into the room to find one child had apparently accidentally shot the other toddler, Gonzalez said at a press conference. The injured child was unresponsive, he said, and the parents immediately called 911.
"The toddler was pronounced deceased at the scene," he said on Twitter.
Officials said it will be up to the district attorney to decide whether charges will be pressed against any of the adults.
"You've got to make sure you're a responsible gun owner," Gonzalez said. "Secure your weapons in a safe place. It's got to be more than just to tell the kids not to touch the weapons."
ABC News' Mark Osborne contributed to this report.