LITTLE ROCK, Ark. -- A 3-year-old boy being taken on a shopping trip by his grandmother was killed in a road rage shooting when a driver opened fire on the grandmother's car because he thought she "wasn't moving fast enough at a stop sign," police said.

The boy and his grandmother were at the stop sign in southwest Little Rock on Saturday evening when a driver apparently angry about the delay stepped out of his car and opened fire, police said. The boy was struck by gunfire at least once, they said.

The grandmother, who wasn't struck, drove away and called police from a shopping centre.

Police arrived at the shopping centre and found the boy in the car outside a JCPenney department store. The boy was taken to a hospital, where he died shortly after, becoming the second young child shot dead in a road rage incident in the city in the last few weeks.

Police Lt. Steve McClanahan said investigators believe the boy and his grandmother "were completely innocent" and have no relationship with Saturday's shooter, who was being sought. He said the grandmother simply was "driving the car and was taking her grandson shopping when the incident occurred."

Police said they were looking for an older black Chevrolet Impala. Police did not release a detailed description of the man who was driving it.

Last month, a 2-year-old girl was killed when a car drove by and someone fired into her vehicle; the shooter in that case hasn't been captured.

Police Chief Kenton Buckner said the road rage killings were frustrating for the police department and the community, especially because the young victims were "very innocent" and "can do very little to protect themselves."

"We cannot have a community to where the least protected among us, being infants, who are dying (in) these senseless crimes in our city," Buckner said.

He said he didn't know if the children's shootings were related.