(CNN)A series of homicides in Stockton, California, are believed to be related, the city's police department says, and officials are offering a $95,000 reward for information in the case.
Five people were shot and killed in separate incidents beginning July 8 through September 27, according to police. All five victims were alone and were shot either in the evening or early morning hours, police said.
It's unclear if the victims were targeted and known to the suspect or if they were selected at random, officials said.
Police said Monday that the owner of a local construction company donated $10,000 toward the reward for infromation, bringing the total to $95,000.
At a news conference Friday, Stockton police said the attacks were not related to robberies or any gang activity in the area.
"It wasn't a robbery, items aren't being stolen, they're not talking about any gang activity in the area or anything. It's just element of surprise," Stockton Police Chief Stanley McFadden said.
The victims have not been publicly identified by police.
Police said four of the victims were Hispanic men ranging from 21 to 54 years old and the fifth victim was a 35-year-old White man.
The city of Stockton and Stockton Crime Stoppers are offering an $85,000 cash reward for information leading to an arrest, according to a Facebook post on Friday.
Police posted an image on Facebook showing the back of a person dressed in all black and said they may be linked to the investigation.
"Please do not fall victim, be alert, have your head on a swivel, stay where it's lit, communicate," McFadden added. "We need you, we need your help, we need your tips, and we need you to help not to spread misinformation."
Stockton is approximately 50 miles south of Sacramento.
CNN has reached out to the Stockton Police Department.