Rewards totaling $85,000 have been offered for information leading to an arrest in five fatal shootings since July in Stockton, Calif., that investigators believe are related, police said.

After reviewing surveillance footage, detectives identified an unidentified “person of interest” in the murders, Stockton Police Chief Stanley McFadden wrote in a department news release. Facebook page Saturday. Police released a grainy photo of the man, taken from behind, dressed in all black and wearing a black beanie.

Police told CBS News there was also evidence of a firearm linking the killings.

The latest killing occurred shortly before 2 a.m. Tuesday, when a 54-year-old man was shot in a residential area north of downtown, McFadden said.

Police said he was the fifth person fatally shot since July 8 within a few square miles. Detectives believe all five murders are connected “based on our investigation and the reports we are receiving,” McFadden said.

“By definition, this could be called a serial killing,” McFadden told CBS News.

Police said the victims were walking alone when they were killed in the evening or early morning in the city of 320,000 about 50 miles south of the state capital, Sacramento. In each case, the chief said, the five male victims were ambushed and shot while alone in poorly lit areas.

The age of the victims is from 21 to 54 years; Four of the men were Hispanic and one was white, McFadden said.

“No family should have to walk around their city in fear or look over their shoulder,” said Stockton Mayor Kevin Lincoln.

“We are committed to protecting our community and solving these cases using every resource at our disposal, including YOU. We need YOUR help!!!! If anyone has information regarding these investigations, please call us immediately. Please remember , that our victims have grieving families of participants who need resolution. If you know something, please tell me,” the executive wrote on Facebook.

The city of Stockton posted a $75,000 reward, and Stockton Crime Stoppers offered an additional $10,000.

Stockton police said they are now looking into other unsolved homicides to see if there is a connection.