Police are hunting for a possible serial killer who may already have struck at least five times in a small California city, officers said Wednesday.

Detectives have released grainy images of a man they want to question over a spate of murders in Stockton, where five people have died in less than three months.

This week they said a sixth killing, in nearby Oakland, which happened last year, could also be connected, reports said.

"It definitely meets the definition of a serial killer," Joe Silva of Stockton Police told AFP on Wednesday.

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Homicide Series Update: Today Chief McFadden released a video of the "Person of Interest" in regards to this series. If you have any info, please call 209-937-8167. There is a $125,000 reward for info that leads to an arrest. pic.twitter.com/QLhcobVFJZ

— Stockton Police Dept (@StocktonPolice) October 5, 2022

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Stockton police chief Stanley McFadden told reporters on Tuesday the five deadly shootings in one area of Stockton between July 8 and September 27 bore a number of similarities.

An April 2021 shooting in Oakland, 80 kilometers away, was also being investigated, police said this week.

McFadden gave no details about the similarities, other than to say they were linked by ballistics evidence.

At least three of the victims were homeless, as was a woman who was shot, but not killed, in April last year in Oakland.

"We're just still trying to figure out if it's one, two or several people that are out committing these series of killings, that we've now been able to trace back to one case to Oakland, California."

Police have released video footage of a man officers want to question, who appears to have a slight limp as he walks past some apartments.

"We don't know who that person is in that video," said Silva.

"That's why we released that video, in the hope that somebody could recognize the person of interest, because they have a very specific walk."

The person in the video was captured on camera close to a number of crime scenes, Silva said.

"Our victims were all alone in dark areas, either late in the evening hours or early morning hours."

Stockton, a city of around 350,000 people, sits 40 miles south of California's state capital of Sacramento.

Silva said the nature of the murders had left the local community "very concerned," and police had scaled up evening and early morning patrols.

"We're just asking for the community to be aware of your surroundings. If you do have to travel at nighttime or the early morning hours, try to bring a friend or a family member and also to avoid dark areas."

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Silva said a reward of $125,000 had been posted for information leading to the capture and conviction of the killer or killers, with hundreds of tips coming into police every day.