(CNN) -- A 16-month-old girl was run over and killed Sunday after authorities say her 4-year-old sibling climbed into a truck and put it in neutral.
Deputies were called to the scene shortly after 1 p.m. (2 p.m. ET), said John Norton, spokesman for the Collin County Sheriff's Office.
The family was loading a pickup to take to a relative's house.
The 4-year-old got the keys to the truck, which was parked in the driveway, climbed in, started it, and put it in neutral, Norton said.
The vehicle rolled forward, running over the 16-month-old girl. She was killed instantly.
The childrens' father was inside the house at the time, Norton said. He heard the truck start and ran to the door. He saw the vehicle rolling forward and ran to stop it.
The truck eventually came to a stop when it struck a neighbor's fence, Norton said.
An investigation is ongoing.