A seven-year-old boy accidentally killed his three-year-old sister after shooting her with an unsecured shotgun in Colorado, police said.
Officers in Westminster, near Denver, said the boy was playing with his sister when he found the loaded gun in Tuesday morning.
The children's mother, Michaela Dawn Harman, was at home at the time and has been arrested on suspicion of child abuse resulting in death.
She is in custody and will be taken to Jefferson County Jail, police said, while the boy and another of his younger siblings are in the temporary care of child welfare officials.
A statement released by Westminster Police said: "While playing, the older brother tragically shot his three-year old sister.
"The girl was taken emergent to a trauma centre by Westminster Fire Department personnel, but her injuries were so significant she did not survive.
"Their mother, Michaela Dawn Harman, and another younger sibling were at the home when the accident happened."
Officers said the incident served as a reminder to parents of the importance of securing firearms.
It added: "Tragic examples like this remind us of the importance to separate guns and ammunition kept in the home and to secure firearms in a locked safe or utilise other child proof safety devices.
"Parents have a responsibility to keep children from being able to access firearms, even inside the home."
