"The three-year-old gained access to a loaded, semi-automatic pistol," Harris County Sheriff Ed Gonzalez said of the shooting late Sunday.

When family members heard a single gunshot, they ran into a bedroom and found the four-year-old girl on the floor "unresponsive," he said.

Gonzalez stressed that the shooting appeared unintentional.

"It just seems like another tragic story of a child gaining access to a firearm and hurting someone else," Gonzalez said.

About 40 percent of US households have guns, according to the Pew Research Center, with most of those also including children.

But less than half of the households with guns store them safely, according to Johns Hopkins University's School of Public Health.

More than 44,000 firearm deaths were reported last year across the United States.

Shootings were a leading cause of death among children under 18 last year, with nearly 1,700 cases, including 314 under 11 years old, according to the Gun Violence Archive.

Texas, with 30 million residents, is one of the easiest places in the country to acquire a gun and they can be carried in public without restriction.

"This is very preventable," Gonzalez said of the shooting Sunday.

"You've got to be sure you're being a responsible gun owner, securing your weapons in a safe place. It's got to be more than just telling young kids not to touch the weapons," he said.