It was last week that local police announced that five men had been murdered under similar circumstances in the city of Stockton, east of San Francisco, between April and September of this year.

Shortly thereafter, another fatal shooting in the nearby port city of Oakland was reported to be linked to the same investigation.

What they have in common is that the people are said to have been shot from an ambush under the cover of darkness when they were alone.

At the same time, an analysis of the bullets that killed the victims has pointed out that there is a connection between the events and that it could thus be the same perpetrator.

"It definitely meets the definition of a serial killer," said Stockton Police Officer Joseph Silva.

"out on a mission"

According to what has emerged in the investigation so far, the victims do not seem to have any relationship to each other.

They are also not said to have been robbed or beaten before being shot to death, nor does anything point to the murders being gang or drug related, according to the police.

- We don't know what the motive is.

What we believe is that it is mission oriented.

This person is out on a mission, said Stockton Police Chief Stanley McFadden, according to Reuters.

The victims, who are both men and women, are aged 21-54.

Another 46-year-old woman who is suspected of having been shot by the same perpetrator survived.

The police have now released a surveillance image of a person who is said to have been at several of the crime scenes.

It shows how he is wearing black clothes and a black cap, and that he is walking through a corridor in an apartment building or a hotel.

Reward has been announced

- The person is interesting to the investigation, says Stanley McFadden, who emphasizes that there is currently insufficient evidence to link the person in the picture to the shooting deaths.

At the same time, a reward of the equivalent of one million kroner has been announced for information leading to an arrest.

According to the description, the person has a limping gait and an "unusually upright" posture.

As of Saturday, no arrests had yet been reported.