The Missouri home where the fatal shooting occurred. (Courtesy of Bill Lindsay/KCTV5 News)

A 9-month-old boy was shot and killed by his 5-year-old brother in a Missouri home on Monday, police said.

The boy found his grandfather’s .22-caliber magnum revolver that was being kept on a shelf built into the headboard in the master bedroom, Nodaway County Sheriff Darren White told The Washington Post. The baby was in a crib in the same room when the gun went off, and a bullet struck him in the head.

“It was just lying out,” White said of the gun. “Little kids are curious. It was in the headboard of the bed, and he found it.”

The children, their mother and two other siblings were reportedly staying in their grandfather’s home in Elmo, Mo., according to a neighbor. White said on Tuesday that it is still unclear whether they lived in the home or were visiting.

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The mother, Alexis Widerholt, was in the kitchen when she heard a loud noise that sounded like a gunshot, according to White. She went into the living room where she found three of her four children — including the 5-year-old boy. Another child was playing with a paintball gun.

Her fourth child, the nine-month-old boy, was inside the master bedroom.

White happened to be near the phone when Widerholt made a frantic call to police just before 9 a.m., pleading for help for her baby, who she thought at the time had been shot in the side of his head with a paintball gun, he said.

When police arrived, they found that the baby was actually suffering from a gunshot wound to the head. Authorities found the .22-caliber gun lying nearby. The boy was rushed by air ambulance to a hospital in Kansas City, where he was pronounced dead.

A neighbor told the Kansas City Fox affiliate WDAF that when she heard the commotion next door, she rushed over and brought the remaining children — who she said are 5, 3½ and 1½ — to her house.

“They’re not very old so they have no clue what was happening, you know,” Kathy Arementrout said.

Police said they have no reason to believe the shooting was anything other than accidental, but it is still under investigation. There were several other weapons in the house, according to White.

“You know, just some gun safety could have come into play, and would have maybe, you know prevented the situation, but it’s just really sad, altogether,” neighbor Jessica Hutchison told local CBS affiliate KCTV.

No one knows exactly how many people are injured or killed accidentally after being shot by a child. Gun control advocates estimate that nine children are unintentionally shot every day in the United States. There are no official nationwide statistics on either phenomenon.

In the small, rural town of Elmo where guns are common, the sheriff says the incident is just a tragic, if cautionary, tale.

“We live in a rural part of the country where firearms are very common and standard, so we pretty much assume that in about every home that we go into, there are firearms there,” White told The Post. “We work off the assumption that everyone that we meet has a firearm sometimes, and that’s okay because it’s legal.”

Speaking to Omaha ABC affiliate KETV, White added: “And most people are very safe with them and this was just one of those cases where everything went together in the wrong way,”

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[This post has been updated.]