A 3-year-old in Houston, Texas, gained access to a firearm and "unintentionally" shot and killed a 4-year-old, Harris County Sheriff Ed Gonzalez said.

Gonzalez said the preliminary investigation showed that the two children had been inside an apartment at 9955 Bammel North Houston Rd. with one other person.

"The toddler was pronounced deceased at the scene," he said on Twitter.

This is a developing story. Please check back for updates.