On a recent summer evening, a young man and a toddler were in a car stopped in a residential neighborhood in Phoenix when gunfire erupted. The two were not hurt, but investigators later said that it slowly dawned on them that they had a larger crime on their hands: A serial shooter had struck again.

The attack on the car carrying the 21-year-old man and the 4-year-old boy on July 11 was the ninth time that an unidentified shooter had preyed on the city’s neighborhoods this year, the police said. Since March, seven people have been killed and two injured.

On Wednesday, the authorities stepped up their efforts to solve the shootings by republicizing an artist’s sketch of the suspect, revealing details about the vehicles he had driven and raising the reward for any information leading to an arrest to $50,000 from $30,000.