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A Missouri prosecutor said Wednesday he has not decided whether to file criminal charges in the death of a 9-month-old boy who was accidentally shot by his 5-year-old brother with their grandfather's handgun.

"There is an active investigation," said Robert Rice, the prosecuting attorney for Nodaway County.

Rice said he could not discuss the specifics of the Monday morning shooting in Elmo, Missouri — including who could face charges — but he was meeting with his team later in the day to review the matter.

Investigators have not yet spoken with the 5-year-old and will need to bring in someone experienced in child interviews if Rice decides it's necessary.

The grandfather, William Porter, told NBC News that he kept the loaded .22 caliber Magnum in a locked gun case, but said it wasn't tamper-proof.

His daughter, Alexis Wiederholt, was visiting with her four young sons on Monday when the eldest somehow got hold of the revolver and fired it. The bullet struck 9-month-old Corbin, who was resting in a playpen, in his head.

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"I'm sorry, Mom. I shot Corbin," the older boy said afterward, according to his mother.

Wiederholt said she had no idea her father had a handgun.

"I didn't know it was there until I turned around and saw it laying on the bed," she said. "I don't know why someone would have a loaded gun in the house while kids were around."

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The Nodaway County sheriff has said the tragedy appears to be an accident but police were not certain how the boy got access to the gun.

The Centers for Disease Control says an average of 62 children are killed in unintentional shootings each year, though Moms Demand Action, a group that lobbies for tougher gun laws, says those figures are under-reported. In the majority of the incidents, the shooter was under 14, the group said in a report last year.

"When one child shoots another it is always due to an adult gun owner’s negligence, and that adult should be held accountable," the group said in a statement in response to the shooting in Elmo.

The group's founder, Shannon Watts, said her group's position is that all guns should be locked up and kept unloaded when they are not being used.

Porter said he did not expect he would be charged because he had locked up the gun. His daughter said she didn't know if he should be charged.

"I'm still trying to process it all," she said. "Right now I'm making arrangements for my child."

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