The tragic incident took place at a home in Gaston, South Carolina, when the boy was looking in the trunk of a car for a toy gun and instead picked up a real loaded gun belonging to his mother's boyfriend.
The gun went off and police said the bullet passed through the car and hit the girl in the chest. Fragments from the shot hit another 5-year-old boy in the arm.
Both injured children were taken to a local hospital, where the girl was declared dead. The boy's injury is not life-threatening, police said.
"They took somebody from me that was so precious. That little girl came in my room, always slept with her momma, she loved me and I loved her," Lynch's mother was quoted as saying by WLTX-TV.
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Lexington County Sheriff James Metts said the five-year-old boy's mother gave him permission to get his toy gun out of the trunk of her boyfriend's car -- not knowing there was a loaded gun inside as well.
"When he was fooling around trying to get his toy gun out, he accidentally pulled the trigger of the real gun while it
Sheriff Metts said the gun should not have been loaded and said a gun lock could have been used.
This is the latest incident involving children who accidentally shoot someone or die of a self-inflicted gunshot wound.
Earlier this month, a two-year-old boy accidentally shot and killed his 11-year-old sister while playing with a handgun in Philadelphia.
In October last year, a babysitter in Vidor, Texas, was charged after a five-year-old toddler she was looking after fatally shot himself with her gun.