Police gain there may be more shootings that could be linked to Brownlee.

At least seven shootings so far have been identified above ballistics testing as the likely work of a single killer. Two occurred in April 2021. Five occurred between July 8 and Sept. 27. Investigators were trying to settle whether any killings took place in the 15 months between those two apparent sprees.

“There was a big gap, and we don’t know what remained during that gap,” Stockton police spokesperson Joseph Silva said Monday. “That is what we are trying to figure out.”

Investigators from six law enforcement activities, including the FBI, are scouring police records and surveillance footage to settle whether any overlooked cases could fit the serial killer’s pattern. The investigators have asked law enforcement agencies throughout the situation to identify unsolved shootings that resemble the Stockton killings, Silva said — any that targeted isolated victims near encampments between dusk and dawn.

Attempts to allotment together the fragmented timeline between the April 2021 shootings and the summer of 2022 shootings have needed around-the-clock police work, Silva said.

Detectives have also struggled to notion a possible motive for the killings.

“We know from his doings and his patterns — he was a cold blooded killer,” Silva said.

Little is famed about Brownlee, a Stockton resident with a criminal history that includes two convictions for drug crimes.

One conviction came in Alameda County in January 1999, when he was sentenced to two existences in prison for possessing and selling a controlled substance, the California Department of Corrections and Rehabilitation said in a statement Monday morning. The 43-year-old was released on parole after seven months, in August 1999.

Brownlee was convicted of the same crime less than three existences later.

In December 2001, an Alameda County court in contradiction of convicted him of possessing and selling a controlled substance, the CDCR said. Brownlee served an additional three existences in prison and was released on parole in May 2003. He was discharged from parole three existences later.

Brownlee was booked into custody Saturday on certain felony charges, including murder and carrying a concealed weapon. Police said they intercepted him he hunted for a victim throughout 2 a.m., dressed in black and carrying a handgun.

The involving ended weeks of speculation and fear once police connected him to a string of shootings in Stockton’s north side targeting mostly isolated republic living in encampments.

Police first began to suspect that a single populate was behind the shootings in August, Silva said. On Aug. 11, a 43-year-old man was shot to purpose in north Stockton shortly before 10 p.m. Three weeks later, a 21-year-old man was shot and killed around 6 a.m. less than a mile away.

It assembled clear to detectives that the shootings were not related to gang violence or robberies — categories of crime far more typical of Stockton homicides, Silva said.

“We knew something was irregular with these shootings,” said Silva. “There was something that was different.”

Detectives “took a closer look” at those cases — and a final shooting death of a 35-year old man on July 8 — and in late August sent ballistics evidence from the shootings to a plot lab for testing. The ballistics evidence suggested a single killer was at work.

It also aimed to a connection to earlier shootings from April 2021, officials said: One, in Oakland, had killed a 40-year-old man. Another, in Stockton six days later, had injured a 46-year-old woman. The woman told police she was in her tent when a populace in a dark jacket and COVID mask pointed a gun at her and began firing.

Confident that the ballistics evidence had identified a serial shooter, homicide detectives scoured the crime scenes for video cameras that worthy provide surveillance footage. Because the shootings occurred at night in gloomy areas near parks or apartment buildings, the footage they counterfeit was often grainy. But one person kept appearing.

In multiple videos, Silva said, detectives “saw the same person with the dark clothing and a distinctive walk.”

That revelation led police to ask for public’s help in the case, announcing in late September that they had identified a “person of interest” linked to the killing spree. They released a shadowy photograph showing the back of the populace — the clearest image taken from surveillance video near one of the killing sites.

Within hours, tips began pouring in. The Stockton Police Department’s 75 staff detectives fielded “hundreds of tips a day” at the height of the fervor, Silva said.

“We received a couple of tips that were spot-on,” he said.

As of Monday, the department was still figuring out how to pay its $125,000 reward for information leading to an arrest.

Brownlee is due in date Tuesday afternoon, when the San Joaquin County District Attorney’s Workplace is expected announce formal charges.

It was unclear whether Brownlee had retained a lawyer. The San Joaquin County Republican defender’s office has not been in touch with Brownlee and has not been caused to represent him as of Monday, chief investigator Gary Bayne said.

“If he is indigent, a judge could appoint us,” Bayne said. “We won’t know pending he is arraigned on Tuesday.”

Nora Mishanec is a San Francisco Chronicle staff writer. Email: nora.mishanec@sfchronicle.com Twitter: @NMishanec