Authorities increased a reward to $40,000 on Monday in the hunt for an Arkansas gunman who killed a 3-year-old child when he apparently opened fire on a car because of a stop sign delay.

The rewards by the city of Little Rock and the FBI come two days after the child, Acen King, was fatally shot while in the backseat of a car being driven by his grandmother, in a shooting described by police as a random act of "road rage."

The shooting was reported outside a shopping center at around 6:20 p.m. Saturday. The child was rushed to a hospital and later died, police said. Audio of a 911 call that was released Monday gave a clearer account of how the shooting unfolded.

Acen King, 3. Courtesy of the family

"Acen has been shot. Oh, my God!” the child’s anguished grandmother, Kim King-Macon, told police in the call.

"I was at the stop sign and the guy blew a horn at me and I blew it back, and he shot, but I thought it was in the air. He shot at the car!" she said in the call.

It is the second time in less than a month that a child was killed in Little Rock after someone opened fire into a vehicle, police said. There have been no arrests in either case.

King-Macon didn’t know her grandson was shot until she got to the shopping center. A police spokesman said that she is "completely innocent" in the incident and that neither she nor the child had any relationship to the shooter.

"It should shock all of our conscience," Little Rock Police Chief Kenton Buckner said. He made an appeal that anyone with information about the shooter contact police.

Police have said they are looking for an older model black Chevrolet Impala and a tall African-American man. The city announced a $20,000 reward Sunday and the FBI on Tuesday also offered a $20,000 reward.

The pastor of the church where Acen was frequently taken, William L. Robinson, struggled Monday to understand the tragedy.

"It’s heartbreaking, and you don’t really have the words and it was so senseless," Robinson told NBC affiliate KARK.