The weapon apparently belonged to the victim's security guard boyfriend.
A toddler riding in the back of a car accidentally shot dead his mother with a gun that slid out from under the driver's seat, US police said.
The 26-year-old victim, Patrice Price, was pronounced dead at the scene following Tuesday's incident in Milwaukee, Wisconsin, the local sheriff's office said.
The child, aged two and a half and identified by local media as a boy, fired the 40-caliber gun through the driver's seat where his mother was at the wheel.
Local broadcaster WISN quoted the victim's father as saying that she was a mother of three.
"This little girl right here meant everything in the world to me," Andre Price told the station. "And I lost the other half of my heart."
The weapon apparently belonged to the victim's security guard boyfriend, with the Milwaukee Journal Sentinel newspaper reporting that she was driving his car because hers had been stolen days earlier.
The sheriff's office said that deputies found a security officer's gun belt belonging to the boyfriend on the floor of the front passenger seat.
The firearm was recovered on the floor behind the driver's seat. An investigation is under way.
Also in the vehicle was another of Price's children, aged one, and the woman's mother. Neither of the children was in a car seat.
Price's mother told relatives that her daughter turned to her after a loud pop rang out and said: "Momma, don't let me die," WISN reported.
Following the shooting, fellow motorists stopped and pushed the car out of traffic before police arrived and performed CPR on Price while awaiting the arrival of emergency medical personnel.
Tuesday's shooting comes on the heels of a similar accident, though not fatal, in Florida last month.
In that instance, a four-year-old boy accidentally shot his mother in the back while she was at the wheel, leaving the passionately pro-gun woman badly wounded.
Jamie Gilt, who had boasted online about her toddler's shooting prowess, was cruising down a major thoroughfare with her son in the back when he shot her on March 8. The bullet went through the seat cushion.
The mother and boy were on their way to pick up a horse, said police, who rushed to Gilt's aid after an officer checked to see why the vehicle had stopped.
The weapon was a .45-caliber handgun the boy found on the floor of the pickup truck Gilt was driving. Police said she legally owned the weapon.