(NewsNation) — The intense search for the person responsible for fatally shooting six men — five in recent months — and hurting one woman in California has continued.

The Stockton Police Department linked the five recent homicides through ballistics tests to two cases from last year — a man’s death in Oakland and the nonfatal shooting of a woman in Stockton.

Stockton Police Chief Stanley McFadden said the string of killings shared similarities since they all have been in dark locations with few cameras, limited lighting and no known witnesses. The chief believes the suspect is on a “mission to kill” since they seem to be catching people by surprise and that no items appear to be taken from the victims.

“We’re finding, you know, the wallets and car keys are just where they lay on the ground. So it’s not robberies or anything. It’s just a person that is killing,” Chief McFadden said.

While appearing on NewsNation’s “Rush Hour” Thursday, true crime author Jillian Lauren says she believes the killings could be sexually motivated. Police say the motive is unknown.

“I think this is a sexually motivated killing in the sense that it’s about power. It’s about pleasure. Whether or not there’s a sexual act involved. This seems like a pleasure motivated killing and how this killer chooses his or her victims is opportunistic,” Lauren said.

Lauren thinks looking at how the victims have been connected — through ballistics evidence — is a key factor in the case.

“I would think that the most important thing they would find would be the murder weapon and connect that then to an owner. There are many reasons that that is an unlikely thing. But with the evidence that I understand they have, that would be the most important, and I know that there is a multi-jurisdictional, as well as a federal task force on this. And I trust they are examining every area of evidence,” Lauren said.

Stockton Mayor Kevin Lincoln expressed that the killings and the suspect on the run have put the city on edge.

“We do know that this person of interest is on a mission. We know that two cities have been impacted: Oakland and the city of Stockton,” Mayor Lincoln said.

Still, Mayor Lincoln said the communities are really stepping up by providing tips to law enforcement with hopes of bringing the killer to justice. He tells NewsNation that authorities are receiving about 100 tips each day and making announcements in multiple languages through several different platforms.

“This is a community effort right here. We can’t do it without the community. We need the community to stay safe. We need the community to be aware of their surroundings, but we also need the community to speak up if they have any information. There’s no tip too big or no tip too small. We need that information for the public so that we can get this person off street,” Mayor Lincoln said.

Police announced that four of the Stockton homicide victims were walking alone, and a fifth was in a parked car when they were killed in the evening or early morning in the city, about 50 miles south of Sacramento. The killings took place between July 8 and Sept. 27 within a radius of a few square miles.

“We have people on foot that have fallen victim, we have people that were in their vehicle that have also fallen victim. We just really need everyone to have their eyes out, their heads on the swivel and protecting each other out there,” McFadden said.

The San Joaquin County Office of the Medical Examiner identified the Stockton victims as Paul Yaw, 35, who was killed on July 8; Salvador Debudey Jr., 43, who died Aug. 11; Jonathan Hernandez Rodriguez, 21, who died Aug. 30; Juan Cruz, 52, who was killed Sept. 21 and Lawrence Lopez Sr., 54.

Investigators released cloudy surveillance footage of their person of interest in the killings. The surviving victim in the case described the suspect as 5’10” to 6’, with a thin build and wearing a dark-colored, hooded sweatshirt with the hood pulled up, dark-colored pants and an all-black “COVID style mask.”

Stockton Police ask residents if they know something to say something. The reward in the case has increased to $125,000.

Anyone with information about the series of homicides is urged to call the Stockton Police Department’s tip line at (209) 937-8167 or email policetips@stocktonca.gov. Tips can also be submitted to Stockton Crime Stoppers at (209) 946-0600.