Beware of Chinese spies offering laptops, women, or educational stipends—and especially watch out for odd LinkedIn requests.

On Tuesday, the Justice Department unsealed new charges against 10 Chinese intelligence officers and hackers who it says perpetrated a years-long scheme to steal trade secrets from aerospace companies. The case continues an impressive tempo from the Justice Department, as it continues to try curb China's massive, wide-ranging, and long-running espionage campaign. In fact, it's the third time since September alone that the US government has charged Chinese intelligence officers and spies, including one of its biggest coups in years: The extradition earlier this month of an alleged Chinese intelligence officer, caught in Europe, who will face a US courtroom.

That arrest marks the first time the US has prosecuted an officer of China's Ministry of State Security. The feds believe that the suspect, Yanjun Xu, spent years cultivating a person he thought was a potential asset inside GE Aviation, which makes closely held jet engine technology.

While historic, the GE Aviation case hardly stands as an outlier. Chinese espionage against the US has emerged over the past two decades as perhaps the most widespread, damaging, and pernicious national security threat facing the country—compromising trade secrets, American jobs, and human lives.

Even as popular culture and public attention has focused in the past decade on a few high-profile cases against Russian intelligence operations, China’s spying efforts have yielded a more steady stream of incidents. Over the last 15 years, dozens of people—including Americans, Chinese nationals, and Europeans—have been arrested, charged, or convicted of economic or military espionage for China. In just the 28-month period that a notorious Russian spy ring unraveled around 2010, US officials charged and prosecuted more than 40 Chinese espionage cases, according to a Justice Department compilation.

The majority of Chinese espionage cases over the years have involved ethnic Chinese, including Chinese students who came to the US for college or advanced degrees, got hired at tech companies, and then absconded back to China with stolen trade secrets. Historically, very few Chinese spying cases have featured the targeting or recruitment of Westerners. But this year has seen a rash of cases of Americans allegedly recruited to spy on China’s behalf, encouraged to turn over sensitive military, intelligence, or economic information—at least one of which started with a simple LinkedIn message.

Sifting through more than a dozen of the major cases that have targeted Westerners, though, provides an illuminating window into how China recruits its spies. The recruitment follows a well-known five-step espionage road map: Spotting, assessing, developing, recruiting, and, finally, what professionals call “handling.”

Stage 1: Spotting

The first step in any espionage recruitment is simply knowing the right people to target. That job often falls to what intelligence professionals call a “spotter,” a person who identifies potential targets, then hands them off to another intelligence officer for further assessment. These spotters, sometimes friendly officials at think tanks, universities, or corporations, are often separate from the intelligence officers who ultimately approach potential spies, allowing a level or two of remove. They sometimes have such “deep cover” that they are considered too valuable to make a recruitment approach directly, leaving that work to a cut-out who could more easily disappear if the recruitment pitch is rejected.

In that vein, last week’s Yanjun Xu indictment ties in to another little-noticed September arrest, where the FBI charged a 27-year-old Chinese citizen and Chicago resident with acting as an unregistered foreign agent for China—the federal criminal charge that prosecutors often use as code for spying. That man, Ji Chaoqun, had arrived in the United States in 2013 to study electrical engineering at the Illinois Institute of Technology, and had subsequently enlisted in the Army Reserves.

This year has already seen a rash of cases of Americans allegedly recruited to spy on China’s behalf.

Yet according to the government’s criminal complaint, Ji Chaoqun had less pure motives at heart than service: He had allegedly been recruited at a Chinese job fair while in college to join a “confidential unit” and work as a “spotter” for Yanjun Xu, helping the MSS officer identify potential recruits and providing background reports on at least eight potential spies. In a 2015 email, Ji Chaoqun wrote that he was enclosing “eight sets of the midterm test questions for the last three years,” according to court documents. He attached eight PDFs of background reports downloaded from sites like Intelius, Instant Checkmate, and Spokeo, which compile public records on individuals for purchase online. (The sites limit purchases to US-based consumers, so they were inaccessible to Yanjun Xu himself.)