(NEW YORK) -- Police are on the lookout for the person responsible for six murders so far in Stockton, California.
Police released only a few details about the string of murders and when they happened: a 35-year-old man fatally shot at 12:31 a.m. on July 8; a 43-year-old man fatally shot at 9:49 p.m. on Aug. 11; a 21-year-old man fatally shot at 6:41 a.m. on Aug. 30; a 52-year-old man fatally shot at 4:27 a.m. on Sept. 21; and a 54-year-old man fatally shot at 1:53 a.m. on Sept. 27.
Another shooting, of a 46-year-old Black woman at Park Street and Union Street in Stockton at 3:20 a.m. on April 16, 2021, was also linked to the investigation, police said. The woman survived her injuries in that shooting, they said.
Police said late Monday that another homicide investigation had been linked to the case: The shooting death of a 40-year-old Hispanic man in Oakland, California, at 4:18 a.m. on April 10, 2021.
Former FBI agent and ABC News contributor Brad Garrett spoke with ABC News Live's Linsey Davis Monday about the cases and his perspective on the ongoing investigation.
ABC NEWS LIVE: Police say that they have physical evidence linking the five killings. Can you give us an example of what might constitute as physical evidence?
BRAD GARRETT: Probably ballistics, which is going to be my guess, because, Linsey, every [case] has been described as people being shot in the back, which would suggest the shooter is not having any physical contact with the victims. So it's not uncommon for serial killers to continue to use the same weapon. So have they ballistically linked the five of them? Perhaps.
ABC NEWS LIVE: Perhaps all of the victims are men, four of them Hispanic, who were all alone and ambushed late at night or in the early morning, just as choice of victims or methods reveal a certain suspect profile or anything about the killer's personality, that that might help track them down.
GARRETT: I'm not sure about that yet. If you look at the locations, they're all in general proximity, not necessarily in the same neighborhoods. This strikes me as an opportunity serial killer. In other words, he's walking around looking for victims. Is he looking for Hispanic males? We don't know that.
The key is with each homicide, can they pull up additional CCTV? Are they going to get lucky and find a witness who sees him get into a car? I can tell you they're really searching the cameras, I suspect, in Stockton to see if they can track him once he leaves the scene, all those little pieces of information. But the absolute thing that will get him caught is him continuing to kill people because he's going to continue to leave clues, which is almost textbook.
ABC NEWS LIVE: If we look at the timetable here, you had more than a month between the first and second killings, but then the fourth and fifth were just six days apart. Does it seem, based on that, that he could be escalating?
GARRETT: Maybe or this may be an opportunity. Was he off from work? Does he work odd hours? Does he not even live or work in Stockton?
Again, all of these little pieces of information become much more relevant as you focus down on an individual. And hopefully, that's going to occur in short order because, I mean, it's just horrible. Five people dead. And this strikes me as a guy that's going to keep going until he gets caught.
ABC NEWS LIVE: And what is that? What strikes you? Is it just rare that somebody who is potentially a serial killer doesn't just stop after five?
GARRETT: You have to understand that they're driven by a compulsion of nothing else, a compulsion to kill people. Sometimes it's just the thrill. And after you kill the first person, presuming the person in July is the first person he's killed, then each one becomes easier. But the high that he gets from committing these crimes is what drives him. And so you will see eventually maybe him going to another victim. They might be close together. They might not be. But it's also somebody that I think has familiarity with Stockton because he's in various neighborhoods. He's probably either walking around, cruising around in a vehicle, or looking for lone individuals when it's dark. That way it gives him less resistance, less likelihood someone's going to see him, and that's really sort of his cover, it appears, with each one of these murders.
ABC NEWS LIVE: Is there something there that might something so small that might help crack a case like this?
GARRETT: Yes. For example, let's go back to ballistics. ATF, the Bureau of Alcohol, Tobacco and Firearms, have a ballistics data set. If this weapon was used in the past, there's a possibility they might be able to link it that way. It's just hard to say, but there are little clues like that. Or the guy makes a slip and he says something to someone. Maybe not admit what he's been doing, but it's just enough for that person to reach out to the police and say, there's something not right. You need to check this guy out.