In southern Kentucky, where children get their first guns even before they start school, Stephanie Sparks paid little attention as her 5-year-old son played with the rifle he was given last year.
Then, as she briefly stepped outside while cleaning the kitchen, "she heard the gun go off," a coroner said.
In a horrific accident that shocked a rural area far from the US debate over gun control, the boy, Kristian, killed his 2-year-old sister, Caroline, with a single shot to the chest on Tuesday (Wednesday NZT).
"Down in Kentucky where we're from, you know, guns are passed down from generation to generation," Cumberland County Coroner Gary White said. "You start at a young age with guns for hunting and everything."
The rifle was made by a company that sells guns specifically for children - "My first rifle" is the slogan - in colours ranging to hot pink and multi-colour swirls.
The boy's rifle was kept in a corner of the mobile home, and the family didn't realise a bullet had been left in it.
The shooting highlights a cultural divide over gun control, which again became a top issue after 20 young children and six adults were shot dead at a Connecticut school in December. It's not uncommon for youths in rural areas of America to own guns for target practice and hunting.
Local newspapers feature photos of children proudly displaying their kills, including turkey and deer.
"It's a normal way of life, and it's not just rural Kentucky, it's rural America - hunting and shooting and sport fishing. It starts at an early age," said Cumberland County Judge Executive John Phelps. "There's probably not a household in this county that doesn't have a gun."
A package of gun control measures proposed by President Barack Obama after the December school shooting has largely failed so far in Congress, with lawmakers pressured by vocal gun owners and the National Rifle Association gun lobby.
Phelps said it had been four or five years since there had been a shooting death in the county.
"The whole town is heartbroken," Phelps said of Burkesville, a farming community of 1,800.
Family friend Logan Wells said he received a frantic call telling him that the little girl was in an accident and to come quickly.
When he got to the hospital, she was already dead. "She passed just when I got there," Wells said.
White said the shooting had been ruled accidental. A police spokesman said it was unclear whether any charges will be filed.
"I think it's too early to say whether there will or won't be," Trooper Billy Gregory said.
White said the boy received the .22-caliber rifle as a gift, but it wasn't clear who gave him the gun, which is known as a Crickett.
"It's a little rifle for a kid. ... The little boy's used to shooting the little gun," White said.
The company that makes the rifle, Keystone Sporting Arms, has a "Kids Korner" on its website with pictures of young boys and girls at shooting ranges and on bird and deer hunts. It says the company produced 60,000 Crickett and Chipmunk rifles for kids in 2008.
Keystone also makes guns for adults, but most of its products are geared toward children, including bright orange vests and hats.
"The goal of KSA is to instil gun safety in the minds of youth shooters and encourage them to gain the knowledge and respect that hunting and shooting activities require and deserve," the website said.
Sharon Rengers, a longtime child advocate at Kosair Children's Hospital in Louisville, said making and marketing weapons specifically for children was "mind-boggling."
"It's like, oh, my God," she said, "we're having a big national debate whether we want to check somebody's background, but we're going to offer a 4-year-old a gun and expect something good from that?"