Three traumatised girls and their two-year-old cousin were being comforted by family after the toddler shot his mother dead as the five shopped in an Idaho Walmart store.
The incident happened when the little boy reached into Veronica J Rutledge’s handbag and her concealed gun fired.
Kootenai County sheriff’s spokesman Stu Miller said that deputies who responded to the emergency call to the store in the town of Hayden, in the north of the state, found the 29-year-old woman dead.
“It appears to be a pretty tragic accident,” he said.
Ms Rutledge’s father-in-law, Terry Rutledge, described her as “a beautiful, young, loving mother. She was not the least bit irresponsible,” he said.
“She was out on what was supposed to be a fun-filled day with her son and nieces.”
Mr Miller said the young boy was left in a shopping trolley, reached into his mother’s handbag and grabbed a small-calibre handgun, which then discharged once. The mother had a concealed weapons permit.
Mr Miller said he arrived shortly after the accident. All the children were taken to a relative’s house.
Ms Rutledge was from Blackfoot in south-eastern Idaho, and her family had come to the area to visit relatives.
She was reported to be an employee of the Idaho National Laboratory, which supports the US Department of Energy in nuclear and energy research and national defence.
Brooke Buchanan, a spokeswoman for Walmart, said in a statement the shooting was a “very sad and tragic accident”.
“We are working closely with the local sheriff’s department while they investigate what happened,” Ms Buchanan said.
Idaho National Laboratory senior chemical engineer Vince Maio worked with Ms Rutledge on a research paper. He said he had been immediately impressed with her.
“She had a lot of maturity for her age,” he said. “Her work was impeccable. She found new ways to do things that we did before and she found ways to do them better.”
There do not appear to be reliable national statistics about the number of accidental fatalities involving children handling guns.
In neighbouring Washington state, a three-year-old boy was seriously injured in November when he accidentally shot himself in the face in a home in Lake Stevens, about 30 miles north of Seattle.
In April, a two-year-old boy apparently shot and killed his 11-year-old sister while they and their siblings played with a gun inside a Philadelphia home. Authorities said the gun was believed to have been brought into the home by the mother’s boyfriend.
Hayden is a politically conservative town of about 9,000 people. Idaho politicians passed legislation earlier this year allowing the carrying of concealed weapons on the state’s public college and university campuses.
Despite facing opposition from all eight of the state’s university college presidents, lawmakers sided with gun rights advocates who said the law would better uphold the Second Amendment, which guarantees the right to bear arms.
Under the law, gun holders are barred from bringing their weapons into dormitories or buildings that hold more than 1,000 people.