Police investigating five fatal shootings in California believe a serial killer might be on the loose in the state.

Detectives released a grainy image of a “person of interest” after the latest killing shortly before 2am on Tuesday.

The surveillance footage image shows an individual from behind who is dressed all in black, with a black hat.

Stockton Police have said they could be a suspect or a witness.

Rewards totalling $85,000 (£76,000) are being offered for information leading to an arrest after the shootings.

Stockton Police Chief Stanley McFadden said the latest victim was a 54-year-old man, who was shot dead in a residential area.

He was the fifth man fatally shot since 8 July within a radius of a few square miles in Stockton.

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Mr McFadden also gave details of each of the attacks that took place.

• 8 July – an unidentified 35-year-old man described only as white was fatally shot at 12.31am
• 11 August – an unidentified 43-year-old man described as Hispanic was killed at 9.49pm
• 30 August – an unidentified 21-year-old man described as Hispanic was fatally shot at 6.41am
• 21 September – an unidentified 52-year-old man described as Hispanic was killed at 4.27am
• 27 September – an unidentified 54-year-old man described as Hispanic was fatally shot about 1.50am

‘Be alert – have your head on a swivel’

Mr McFadden said he believes all five homicides are related “based on our investigation and the reports we are receiving”. All the victims were walking alone when they were killed.

Stockton is a city of 320,000 residents about 50 miles (80km) south of the state capital of Sacramento.

Mr McFadden said police have ruled out gang violence, hate crime, and the targeting of homeless residents, with detectives not yet able to determine if there is one or more shooters.

The police chief wrote on Facebook: “We are committed to protecting our community and solving these cases, utilizing all the resources at our disposal including YOU. We need YOUR help!!!! If anyone, has information regarding these investigations, call us immediately.

“Please remember our victims have grieving family members who need resolution. If you know something, say something.”

Mr McFadden added the killings have taken place in “dimly lit” areas “overshadowed by trees”.

“Be alert,” he said. “Have your head on a swivel.”

The city of Stockton put up a $75,000 (£67,000) reward and Stockton Crime Stoppers offered an additional $10,000 (£8,884).