Reports of smash-and-grab robberies of watch stores kept popping up around Southern California. On January 2, 2016, three men burst into Ben Bridge Jeweler in Santa Monica. They attacked the glass cases with sledgehammers but found them difficult to break. Flustered, they fled empty-handed, escaping in a burgundy Chevy Tahoe.

Two weeks later, five men in hooded sweatshirts and masks entered a watch boutique in South Coast Plaza, an Orange County mall. They smashed a glass case, grabbed 133 Rolex watches worth more than $2 million, and fled. About a mile from the mall, police found an abandoned black Chevy Impala containing evidence related to the crime.

Stearman had wrapped up his gun-store investigation and was ready for another major case. Most robberies are investigated by local law enforcement and prosecuted in state court, but these seemed organized, which could mean conspiracy to interfere with commerce by robbery—a federal crime. The heists also seemed meticulously planned, right down to repeated use of stolen Chevys as getaway cars. Stearman knew repeat offenders could be almost ritualistic, mimicking past offenses: passing the bank teller the same note, say, or always wearing the same shirt or hat. Were late-model Chevys a signature?

Curious, Stearman searched for similar robberies on Google. He saw a story about the watches strewn on the 405 freeway and another, about the Century City heist. He also read about a December 2015 robbery in which three masked men had used hammers and axes to smash cases in a Ben Bridge Jeweler in Riverside County. Their getaway car: a stolen Chevy Impala.

Stearman had been with the ATF for 10 years, ever since he graduated from college. He had taken to the bureau instantly. The special agents—the ATF has just over 2,600, one-fifth that of the FBI—weren't flashy or self-important. Above all else, they seemed to put a premium on competence. “There wasn't a lot of uptightness,” he said.

That was a plus for a guy who shuns neckties except for when he has to go to court. When I first sat down with him, he wore his preferred uniform: an untucked T-shirt, jeans, and Chuck Taylors. His casual look, however, belied a furious work ethic. Jeff Chemerinsky, an assistant U.S. attorney who worked on the watch-heist case, described Stearman as relentless.

“I obsess over these cases,” Stearman told me. “I always feel like I'm right behind the criminals and if I stop, they get further ahead.”

What inspired Stearman to take on the case, more than the robbers' seeming fondness for Chevys, was imagining the suffering of those who witnessed the crimes. “I don't want my family at a mall where this stuff is going on,” he told me. These robberies were a form of street terror. It was time for them to stop.

Smash-and-grabs are designed for speed. It's rare for one to last longer than two minutes, and most are over in half that. Masks and gloves obscure the robbers' identities, so unless they leave DNA behind, there are often few leads to follow. This was just one of the challenges Stearman faced. Another: He didn't want to catch only the men who entered the stores. If these crimes were connected, as he suspected, there were ringleaders calling the shots. He wanted to find them, too.

Stearman was optimistic. “This isn't rocket science,” he said. “We're talking about guys who are greedy. They're gracious enough to keep giving me the clues I need.”

The robbers were hard at work. On January 22, 2016, they hit a family-owned jewelry store in an outdoor mall in Topanga Canyon, stealing three Rolexes, one Omega, one Tissot, and one Longines, worth more than $190,000. Two weeks later, on Super Bowl Sunday, an employee of a Ben Bridge Jeweler in Thousand Oaks was on the phone giving her daughter a recipe for chicken wings when two masked men burst in and stole 35 watches worth nearly $300,000. The suspects fled in a black Chevy Suburban.