"We don't have any video of anybody holding any gun or actually committing a crime," Silva said in a phone interview Monday.
Still, the available footage, as well as ballistics evidence, link the five killings, he said. All five men were shot by a handgun, though it's not yet clear if it was the same gun used in each crime.
"It definitely meets the definition of a serial killer," Silva said. "What makes this different is the shooter is just looking for an opportunity and unfortunately our victims were alone in a dark area."
There may even be multiple people involved in the violence. "To be honest, we just don't know," he said.
"This person or people who are out doing this, they are definitely very bold and brazen."
Police said the victims were each walking alone or in a parked car when they were killed in the evening or early morning in the city of 320,000 residents about 80km south of the state capital, Sacramento.
The ages of the victims range from 21 to 54; four of the men were Hispanic and one was white.