The boys were in the bathtub in their family's home Tuesday evening when their mother stepped away. The younger boy left the bathtub, found the rusty rifle and shot his brother in the lower back while playing with the weapon, Houston Police Department police said.
The victim was hospitalized with what police said were non-life threatening injuries. No identities were released.
The boys' father returned home shortly after the shooting, and was questioned, the Houston Chronicle reported Wednesday.
Police said the parents may face charges for leaving the weapon unsecured.