In the magazine earlier this month, I wrote about Greg Chung, a Chinese-American engineer at Boeing who worked on NASA’s space-shuttle program. In 2009, Chung became the first American to be convicted in a jury trial on charges of economic espionage, for passing unclassified technical documents to China.

While reporting the story, I learned a great deal about an earlier investigation involving another Chinese-American engineer, named Chi Mak, who led F.B.I. agents to Greg Chung. The Mak case, which began in 2004, was among the F.B.I.’s biggest counterintelligence investigations, involving intense surveillance that went on for more than a year.

The stakes were high: at that time, the F.B.I. did not have a stellar record investigating Chinese espionage. Three years earlier, the government had been publicly humiliated by its failed attempt to prosecute the Chinese-American scientist Wen Ho Lee on charges of passing nuclear secrets from the Los Alamos National Laboratory to China, in a case that came to be seen by some observers as an example of racial prejudice. The investigation of Chi Mak—followed by the successful investigation and prosecution of Greg Chung—turned out to be a milestone in the F.B.I.’s efforts against Chinese espionage, and demonstrated that Chinese spies had indeed been stealing U.S. technological secrets.

While Chung volunteered his services to China out of what seemed to be love for his motherland, the F.B.I. believed that Mak was a trained operative who had been planted in the U.S. by Chinese intelligence. Beginning in 1988, Mak had worked at Power Paragon, a defense company in Anaheim, California, that developed power systems for the U.S. Navy. The F.B.I. suspected that Mak, who immigrated to the U.S. from Hong Kong in the late nineteen-seventies, had been passing sensitive military technology to China for years.

The investigation began when the F.B.I. was tipped off to a potential espionage threat at Power Paragon. The case was assigned to a special agent named James Gaylord; since the technologies at risk involved the Navy, Gaylord and his F.B.I. colleagues were joined by agents from the Naval Criminal Investigation Service. Mak was put under extensive surveillance: the investigators installed a hidden camera outside his home, in Downey, California, to monitor his comings and goings, and surveillance teams followed him wherever he went. All of his phone calls were recorded.

A short and energetic sixty-four-year-old with a quick smile, Mak was a model employee at Power Paragon. Other workers at the company often turned to him for help in solving problems, and Mak provided it with the enthusiasm of a man who appeared to live for engineering. His assimilation into American life was limited to the workplace: he and his wife, Rebecca, led a quiet life, never socializing with neighbors. Rebecca was a sullen, stern woman whose proficiency in English had remained poor during her two and a half decades in the United States. She never went anywhere without Mak, except to take a walk around the neighborhood in the morning.

Sitting around the house—secret audio recordings would later show—the two often talked about Chinese politics, remarking that Mao, like Stalin, was misunderstood by history. The influence of Maoist ideology was, perhaps, evident in the Maks’ extreme frugality: they ate their meals off of newspapers, which they would roll up and toss in the garbage. Every Saturday morning, after a game of tennis, they drove to a gas station and washed their car using the mops and towels there. From the gas station, the Maks drove to a hardware store and disappeared into the lumber section for ten minutes, never buying anything. For weeks, the agents following them wondered if the Maks were making a dead drop, but it turned out that the lumber section offered free coffee at that hour.

One evening in September, 2004, Gaylord drove to a playground next to the freeway in Downey. About two dozen of Gaylord’s colleagues from the F.B.I. were already gathered there, including a team from the East Coast that specialized in making clandestine entries into the homes of investigation suspects. That night, they planned to conduct a secret search of Mak’s house. Mak and Rebecca were vacationing in Alaska, and this gave agents an opportunity to use a court order authorizing them to enter the Maks’ residence in their absence.

For weeks, agents had been watching Blandwood Road, the street the Maks lived on, researching the nightly patterns of nearby neighbors. The person next door routinely woke up at three to go to the bathroom, walking past a window that offered a partial view into the Maks’ house. Behind the Maks’ residence was a dog that was given to barking loudly. A neighbor across the street came out every morning at four to smoke a cigarette. If any of them were to raise an alarm, the search would not remain secret. Mak would find out and, if he was indeed a spy, it would become harder to find evidence against him.

Shortly before midnight, Gaylord and two other agents got into a Chevy minivan with the middle and back rows of seats removed. The vehicle was identical in appearance to the one that Mak drove; it would raise no suspicions even if neighbors happened to notice it. The agents lay down flat in the back of the van, leaving only the driver visible from the street. After getting the go-ahead from a surveillance team, the van pulled out from the playground and drove to Blandwood Road, stopping a short distance from the Maks’ house.

The group of entry specialists was already inside the house. Gaylord gently opened the front door and entered, letting two other agents in behind him. The men stood motionless, waiting for their eyes to adjust to the darkness. Everything they could see was covered in a thick layer of dust, including a model airplane on a coffee table and a vacuum cleaner in the hallway. In the dim light, Gaylord saw stacks of documents, some two to three feet high, everywhere: by the front door, on the dinner table, in the home office.

The agents began photographing the documents, taking care to put them back exactly as they had been. Among the stacks were manuals and designs for power systems on U.S. Navy ships and concepts for new naval technologies under development. One set of documents contained information about the Virginia-class submarines, describing ways to cloak submarine propellers and fire anti-aircraft weapons underwater.

The agents took pictures of other materials: tax returns, travel documents, and an address book listing Mak’s contacts, including several other engineers of Chinese origin living in California. This is where the F.B.I. first came across the name Greg Chung.

The F.B.I. was also watching Chi Mak’s younger brother, Tai Mak, who had moved to the U.S. from Hong Kong in 2001. Tai was a broadcast engineer for a Hong Kong-based satellite-television channel, Phoenix TV, which is partly owned by the Chinese government. He lived in Alhambra, about twelve miles from Downey, with his wife, Fuk Li, and their two teen-age children, Billy and Shirley.

Fuk and Rebecca didn’t get along, and would bad-mouth each other to their husbands. Still, the two families got together every few weeks, usually at a Chinese restaurant in Alhambra, which has a large Chinese-American population. A frequent topic of conversation was Fuk’s aging mother, who lived alone in Guangzhou. Fuk and Tai were concerned about her health, and they depended on a family friend named Pu Pei-liang, a scholar at the Center for Asia Pacific Studies at Sun Yat-sen University, to check on her periodically.