Greg Chung was eager to help China. “He has a big heart,” his wife said. Family Photograph Courtesy FBI

Greg Chung was at home on February 1, 2003, when the space shuttle Columbia fell from the sky. His son Jeffrey called to tell him the news: the ship had broken apart while returning to Earth, and all seven astronauts on board had died. “That’s not a good joke to make,” Chung said. An American citizen who was born in China, Chung lived with his wife, Ling, on a cul-de-sac in Orange, California. Until his retirement, a few months earlier, he had worked on NASA’s space-shuttle program. Among other things, he had helped to design the Columbia’s crew cabin. When he realized that Jeffrey was telling the truth, he hung up the phone and wept.

In 1972, NASA outsourced the design and development of its space shuttles to the Rockwell Corporation, which was later acquired by Boeing. For three decades, Chung was a structural engineer in the stress-analysis group. The work was repetitive, but he was well suited to it. He rarely left his office, even for coffee; instead, he sat at his desk, running computer models that predicted how the fuselage would hold up under various intensities of heat and pressure.

After the Columbia accident, NASA asked Boeing to improve the design of the next shuttle. Chung had been one of the best analysts in his group, and his former supervisor called to hire him back as a subcontractor. Though he was seventy, he was glad to postpone retirement. He returned to his former habits, coming home late for dinner and then working until midnight. He was driven not by the prospect of a promotion or a raise but by the pleasure of the work. “He’d tell me how much money he had saved for Boeing,” Ling told me later. “I always teased him: ‘your Boeing, your Boeing.’ ”

In April, 2006, two F.B.I. agents visited Chung at home. He had designed the house in Orange, and it included a deck that he and Ling had built themselves. In the large front yard, Chung had planted lemon trees and a tomato patch, which he sprinkled with water recycled from the shower. Their two sons—Jeffrey and his older brother, Shane—lived nearby with their families.

Chung, a tall man with a lean, impassive face, invited the agents inside. They asked him about Chi Mak, an acquaintance of Chung’s, who had been arrested several months earlier. Mak had moved to California from Hong Kong in the seventies, and had worked as an engineer at Power Paragon, a company that builds power-distribution systems for the Navy. For years, China had been trying to modernize its naval fleet, and the F.B.I. suspected that Mak had been trained by Chinese intelligence services and sent to the United States as a spy.

For more than a year leading up to Mak’s arrest, F.B.I. agents had tapped his phone and followed him on his errands. Once, while Mak and his wife were on vacation in Alaska, agents entered their house in the middle of the night. They were careful to leave no trace—even the cobwebs in the living room remained intact after the search—as they photographed hundreds of Mak’s documents, including his address book, in which they found the names of several Chinese-American engineers. One of the names was Greg D. Chung.

Chung, whose given name was Dongfan, had gone by Greg since arriving in the United States, forty years earlier. He told the F.B.I. agents that he and Ling went out to dinner with the Maks once or twice a year, but that, because Chi Mak was an electrical engineer, not a structural engineer, the two men never discussed work. The agents thanked Chung and left. They had learned a few useful pieces of information, but nothing that implicated him in any wrongdoing.

A few weeks later, F.B.I. agents conducted another search of Mak’s house. In a stack of old bank statements, they found a photocopied letter, written in Chinese on the stationery of a Beijing hotel, from Gu Wei Hao, an official in the Chinese aviation ministry. It was dated 1987, and it was addressed not to Chi Mak but to Lingjia and Dongfan Chung.

In his letter, one of several documents that the F.B.I. recently shared with me, Gu asked Chung to collect information that would help China develop its space program. The Chinese government had embarked on a plan to build an Earth-orbiting space station, and Gu was looking for any relevant technical knowledge. “For all the expenses that you incur in collecting or purchasing information, I will find a way to pay you cash in person, and you will be allowed to carry it outside the country,” Gu wrote. He invited Chung to Guangzhou, where they could discuss technical matters “in a small setting” that would be “very safe.” Because Chung was an American citizen, Gu advised him to apply for a tourist visa; on the application, he should claim to be “visiting relatives in China.” Gu concluded, “It is your honor and China’s fortune that you are able to realize your wish of dedicating yourself to the service of your country.”

Chung was now an espionage suspect. The F.B.I. opened a new investigation, under the direction of an agent named Kevin Moberly, an athletic man in his early forties with cropped hair and a neat goatee. One night in August, 2006, Moberly woke up at 2 A.M. and got dressed. He and another agent, Bill Baoerjin, drove to Orange and parked on Grovewood Lane, less than a hundred yards from Chung’s house. They sat in the car for twenty minutes, scanning the neighborhood and letting their eyes adjust to the darkness. Then, using flashlights covered with red filters to make the beams less conspicuous, they rifled through two trash cans outside Chung’s gate. They found a bundle of Chinese-language newspapers, which they took back to the office.

Slipped between the newspaper pages were several technical documents from Rockwell and Boeing. Moberly, who had been an intelligence officer in the Air Force before joining the F.B.I., recognized the abbreviations: “O.V.,” for Orbital Vehicle; “S.T.S.,” for shuttle transportation system. There was no evidence that Chung was attempting to make a dead drop. He seemed simply to be getting rid of sensitive documents, possibly as a reaction to the Mak case, which had been in the news for months.

Moberly and Baoerjin returned the next week for another search. This time, a neighbor’s car passed by at 4 A.M., and the agents had to duck behind the trash cans. Moberly decided that curbside rummaging was too risky. He made an arrangement with the trash collectors: after the garbage truck left the Chungs’ neighborhood but before it reached a processing center, it would stop at an agreed-upon spot; F.B.I. agents would remove the trash they needed without divulging which house was under investigation.

The following week, shortly after sunrise, Chung wheeled out a large recycling bin and placed it next to the two trash cans, which he had put out the night before. He then stepped behind the bushes in his front yard and waited for a minute, watching the street, before he returned to the house. When the investigators retrieved the contents of the recycling bin, they found more than six hundred pages from Boeing, full of graphs and line drawings. The words “proprietary” or “trade secret” appeared on several of the pages.