The Madera County sheriff's lieutenant was getting ready for work when he placed the gun on his bed in his Fresno home just on Friday morning, Lt. Joe Gomez said. Told the Fresno Bee.
When he went into the bathroom, his 10-year-old daughter walked into the bedroom and grabbed the gun, Gomez said.
She "accidentally manipulated it," discharging the only round in the chamber and hitting her younger sister in the lower torso, he said.
Police declined to release more details, citing an ongoing internal administrative investigation.
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The deputy, who has not been named, will not be placed on leave because he left the gun unattended for only a short time, Lt. Bill Ward of the Sheriff's Office said.
"He will be allowed any time off he needs to participate in the healing of his daughter and in winding through the cycle of what happens when there's a tragedy," Ward said.
The Fresno County District Attorney's Office will decide whether to file charges against the deputy, Ward said.
On January 31, a three-year-old boy, who while playing with a handgun had accidentally shot his parents in a hotel room in New Mexico.
The boy removed a handgun from his mother's purse and fired one shot, striking his father in the lower backside.
In yet another shooting incident in the US involving a toddler, a 2-year-old boy accidentally shot and killed himself with his father's gun in Florida.
A 9-year-old accidentally killed a shooting instructor in August last year while firing a submachine gun at an Arizona range.