A suspected California serial killer appears to be “on a mission” police have said, although they admit they are still baffled as to what that mission is.

The gunman has so far shot dead six men and wounded a woman in a 18-month killing spree in the Stockton and Oakland areas.

There is a $125,000 (£109,000) reward for information leading to an arrest.

The victims appear to have little in common, no particular ethnic group seems to have been targeted and while some were homeless, others were not. None were beaten or robbed and they don’t appear to have known each other.

“We don’t know what the motive is. What we do believe is that it’s mission-oriented,” Stockton Police Chief Stanley McFadden said Tuesday. “This person’s on a mission.”

The first victim was 39-year-old Juan Vasquez Serrano, who was shot multiple times in Oakland in April last year.

Days later the woman was shot and although wounded managed to scare off the attacker by walking towards him.

She described him as wearing a dark-coloured hooded sweatshirt with the hood pulled up, dark-coloured pants and an all-black COVID-style face mask.

The killer than appears to have laid low for for more than a year before switching his attention to the Stockton area, around 70 miles from Oakland.

On 8 July this year Paul Yaw, 35, was shot dead, followed by Salvador Debudey Jr., 43, on 11 August; Jonathan Hernandez Rodriguez, 21, on 30 August; Juan Cruz, 52, who died on 21 September.

The most recent victim was Lorenzo Lopez Sr., 54, who was shot dead on 27 September.

He “was just a person who was out here at the wrong place, at the wrong time, at the wrong circumstance,” his brother, Jerry Lopez, told local television station KXTV-TV. “It’s hard to process that this has happened.”

Stockton Police said four of the victims were walking alone while the fifth was sitting in his car.

It is believed the same gun was used for both the Oakland and Stockton shootings.

Police admit they have little to go on apart from some indistinct CCTV footage.

“To be honest, we just don’t know,” Stockton Police Officer Joseph Silva said. “This person or people who are out doing this, they are definitely very bold and brazen.”