Over the past three days, police and residents in four Bay Area cities reported separate clusters of serial vehicle break-ins, sharing images of car after car sitting in broken glass in a troubling reminder of a problem that has plagued the region for the past decade.

Monday morning, residents in San Jose’s Willow Glen neighborhood reported at least a dozen car burglaries. Around the same time, five teens were arrested in connection with eight smash-and-grab car burglaries in Palo Alto. On Tuesday morning, a 16-year-old and 19-year-old were arrested following a high-speed police chase on suspicion of swiftly and methodically breaking into 30 cars in Fremont. The following day, Sunnyvale police officers arrested two men in connection with 43 car break-ins that occurred in a two-hour span Tuesday night.

But investigators cautioned that it is too early to determine if the incidents were linked.

“We’re certainly in touch with these surrounding agencies, but I’m not comfortable saying they’re definitively connected,” Sunnyvale Department of Public Safety Capt. Dan Pistor said Wednesday.

Together, though, the cases underscore a persistent rise in car burglaries that has plagued many Bay Area cities, with police agencies across the region making little headway in their efforts to combat thieves, including by going after the buyers, so-called “fences,” who turn the stolen goods — primarily electronics — into instant cash.

From 2009 to 2018, nine of Santa Clara County’s 13 incorporated cities saw an increase in car burglaries, including a 22 percent rise in San Jose, to 6,208 reported incidents, according to figures maintained by the state Department of Justice. Palo Alto saw a 37 percent increase in that same period, to 603 incidents in 2018. Sunnyvale was an exception, recording a 24 percent drop in car break-ins during the same stretch.

In Fremont, police have projected a 25 percent increase in car break-ins for 2019, nearing the 2,078 break-ins the city recorded in 2017, a year that was a modern peak for such crimes statewide. Oakland, meanwhile, was tracking a 37 percent increase in 2019. And though San Francisco was on pace to see a 4 percent drop in 2019, the issue of car break-ins there remains so ubiquitous that city officials have proposed amending state law to make it easier to prosecute car burglars.

“It’s incredibly lucrative and easy to flip electronics. You can get $200 cash in hand for certain devices. It’s attractive and low risk and is behind the huge spike we’ve been experiencing,” said Marisa McKeown, a Santa Clara County supervising deputy district attorney who leads her office’s Crime Strategies Unit.

Local car burglars have apparently extended their footprint across the state: Los Angeles police blame gang crews from the Bay Area for a spike in vehicle break-ins targeting Southern California tourists, according to the Los Angeles Times.

Some investigators believe the recent smash-and-grab sprees in the Bay Area were similarly carried out by gangs who had traveled from other parts of the region, a practice known as “bipping,” said Lt. Shawn Pate, a supervisor on the Safe Streets Task Force, run out of the Contra Costa County District Attorney’s Office.

“There’s a very strong likelihood that this is the work of gangs,” Pate said.

The gangs travel in rental cars to avoid detection, he said, and can make up to five figures in cash a day. The strategy first became a money-maker for gangs about six years ago, he said, but really began to spike in 2016.

In recent years, police agencies have coordinated to go after the fencing operations that allow gang crews to easily monetize their thefts.

Last fall, a major fencing operation run out of Hayward and El Cajon was broken up after an investigation led by Fremont police, which yielded $350,000 in cash and 1,800 stolen electronic devices, most of which were headed to places such as Dubai, Mexico, China, Russia and Iran. A 2018 operation run by the Fremont police and the Santa Clara County District Attorney’s REACT tech-crime task force — dubbed “Million Dollar Baby” — uncovered an alleged scheme in which thousands of laptop computers stolen during vehicle break-ins were bought in cash by a broker and eventually shipped to Vietnam.

“Some of our most effective work has been in cutting off the funding streams,” McKeown said.

San Jose police Lt. Greg Lombardo, who oversees the SJPD Burglary Prevention Unit, said it’s too soon to tell what larger influences might be behind the week’s strings of car burglaries but noted that they are typically the work of gangs or teen vandals looking for quick cash.

Lombardo said that break-ins in residential areas — like those reported in Willow Glen on Monday — are not usually the hallmark of the gang crews, who he said typically target high-density areas like shopping malls and public parking lots. He noted that seasonal timing, in the wake of the holidays, also could be a factor.

“There are still lots of people going to shopping malls to return gifts, and people are leaving their stuff in cars,” he said.

He added that preventative measures by vehicle owners are critical to battling car break-ins. Besides keeping valuable items out of plain sight — or taking them out of the car altogether — he said residents now have to be wary about storing items in their car trunks before parking in a public area and be mindful of whether items, even hidden, are emitting Bluetooth signals that can be detected by thieves.

“We can spend all day investigating crimes,” Lombardo said. “But if people take proactive steps to prevent crime, it can make a big difference in reducing those incidents.”

That elicited a lukewarm response from De Anna Mirzadegan, treasurer of the Willow Glen Neighborhood Association, who said she’s heard from various members of her organization in recent weeks who have suffered car break-ins.

Beyond the irritation of losing important belongings such as a phone or a passport, residents feel a sense of resignation, Mirzadegan said.

“People are really frustrated and feel like not enough is being done — that it’s just kind of, ‘This is is the way it is, deal with it,’ ” she said. “It’s your job to protect yourself.”