The outlet quoted him as saying that there were five adults at an apartment on Bammel North Houston Road near Tomball Parkway where the incident took place. They were all family and friends, Mr Gonzales added. According to law enforcement authorities, the girl lived at the apartment with their parents.

"@HCSOTexas units responded to an apt unit at 9955 Bammel North Houston. Preliminary info: two toddlers were inside the unit with other persons. One toddler (3 yrs) gained access to a firearm and unintentionally shot the other (4 yrs)," Mr Gozales said in his tweet.

"The toddler was pronounced deceased at the scene. Investigators and PIO are enroute," he added.

The parents of the girls left them alone in one room and that's where the three-year-old found a loaded gun, according to the sheriff. The toddler fired a single shot, which killed her elder sister, the officer added.

According to the police, family members heard the shot, secured the gun and called the emergency number 911.

The shooting appears to be accidental. It's unclear if any charges will be filed in connection with the shooting.