(NewsNation) — Police in Stockton, California, believe they have a serial killer on their hands, and they are asking the public for their help.

Five murders this year between July and last week are now linked by ballistics to at least two other attacks from last year.

Police say they are dealing with one or more killers who case their next location ahead of an attack, always picking a dark, desolate area with few to no cameras, catching the victims who are always alone and off guard.

In April 2021 a 40-year-old Hispanic man was killed in Oakland, the only one of these cases outside of Stockton. One week later, a 46-year-old woman survived an attack.

“This was a Black female adult, she was in her tent when she heard someone walking by her campsite. When she came out her tent, she encountered someone holding a gun at her,” said Stockton Police Chief Stanley McFadden.

The woman described the suspect as 5-foot-1 to 6-feet, with a slim build, wearing dark clothes and a black COVID-style mask. Police say they’re looking closely at his walk and his unusual cadence.

“We also want you to notice how upright this person is, we like to think we all have a normal gate when we walk around, but this person’s posture is noticeably upright,” McFadden said.

Race isn’t necessarily considered a motive. While five of the seven victims were Hispanic, 40% of Stockton is Hispanic.

Police have ballistics that link all these shootings together. They’ve ruled out that these are gang-related killings and robberies. Keys, wallets and cars have all been left behind at the scenes.

Retired FBI Special Agent Jeffrey Rinek, along with police, think the killer is finding places to attack during the day, before striking late night or early morning.

“(The suspect) might be doing a little surveillance, so there won’t be people nearby to see,” Rinek said.

Stockton police aren’t ruling out the possibility that there could be more victims. They have a task force working old cases to see if there are any more ballistic matches. In the meantime, police have increased night patrols.