The 53-year-old grandfather of the victim has been arrested for child endangerment, KABC TV reported.
The shooting happened around 9 a.m. at a home in Duffy Street in San Bernardino's Muscoy area on Friday, the sheriff's office told the media.
A neighbour said: "When they brought her out, she had a little patch right here over her chest. It didn't look like a big hole. They were pumping her as they put her in the ambulance."
The boy allegedly involved in the shooting was also visibly distraught as he was carried out of the home, KABC TV reported.
She died about an hour after being hospitalised. The adults of the house were taken to the Sheriff' s Department headquarters for questioning, the San Bernardino Sun reported.
After several interviews, investigators determined Cesar Lopez, the victim's grandfather, had left the gun in a place accessible to kids.
Lopez was arrested and will be booked for child endangerment and felon in possession of a firearm, the sheriff's office said.
Neighbours termed the incident "devastating" and said people who got guns have to lock them up away from kids.
In 2018 alone 7,971 people have died in the US due to gun violence, of whom 381 are children, not for profit organisation gunviolencearchive.org said.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
