On nights when Xiaoxing Xi can't sleep, his mind races through the possibilities of what may have started it all.
He thinks back to emails that could have prompted the FBI probe. About conversations that might have drawn the armed agents to his home. Through the events that left him publicly labeled a Chinese spy.
It's been nearly a year since federal prosecutors accused Xi, a world renowned Temple University physicist, of selling scientific secrets with potential military applications to China. It's been eight months since they abruptly dropped all charges - with little explanation.
After all this time, Xi still has few answers.
By all appearances, he has begun to regain the trappings of a normal life. He's back on the fifth floor of Temple's Science Education and Research Center, in a spacious corner office with tall windows and a sweeping campus view. He's refocused on the federally funded research he was forced to abandon. He's teaching again.
Yet he's still missing perhaps the most important thing that marked his life in America before his arrest: peace of mind.
"We cannot get rid of the thought that the FBI is reading every one of our emails and listening to our phone conversations to find something," Xi, 58, said in one of his first extensive interviews. "I am determined to move on, but that's there."
Xi's journey from leading scientist to accused spy mirrors the experience of other Chinese American scientists, each on the wrong end of the Obama administration's push to halt the state-sponsored siphoning of American trade secrets. It's also fueled new concerns over racial profiling and the misguided targeting of academics.
Born in Beijing and raised at the height of China's Cultural Revolution, Xi (pronounced Zschee) came to America with his wife in 1989, excited by research opportunities and the prospects of a better life. He became a naturalized citizen, joined universities across the Northeast and gained prominence in his field. After more than a decade at Pennsylvania State University, he joined Temple in 2009.
But the couple's culture and memories run deep. In their homeland is a saying - "to pick the bones from an egg" - that the Chinese use to describe a search for something that simply isn't there.
Xi uses it while talking about the government's attempts to build a case against him. His wife, Qi Li, who goes by the name "Jenny," uses it to describe her own memories from the Cultural Revolution, a period of chaos and violence as Mao Tse-tung attempted to purge China of capitalist ideals.
She was 6, she said, when Mao's supporters raided her home, took her furniture, burned photos and evicted her family.
"The government seemed to be doing whatever they wanted to do," she said. "When you're dealing with the government, it's very frightening and confusing."
So when U.S. agents stormed the couple's Lower Merion Township home nearly 50 years later, it seemed all too familiar.
It was around 6:30 a.m. on May 21, 2015 when FBI agents came to the stylish, tree-lined street in Penn Valley, bulletproof vests on and guns - even a small battering ram - in tow, Xi recalled.
The seconds that followed were a blur: Xi was handcuffed and pushed against a wall, he said, while agents pointed guns at his wife and children upstairs.
Unclear why he was being arrested or what he was charged with, he said, he was taken to a Philadelphia office, while his family remained behind for questioning, their house later searched and stripped of laptops, cellphones, hard drives and bank statements.
"Of course, I thought this must be a mistake," Xi said.
But then he was taken into a room at the FBI, handcuffed to a table, and questioned by agents.
"I had the realization that they were going to put me in jail," Xi said. "If I weren't at home, what would my wife do?"
Navigating the justice system proved confusing and unfamiliar. Xi saw no problem answering questions without a lawyer present.
Asked whether he collaborated in China, taught Chinese students, or visited Chinese labs, he answered "yes," Xi recalled. International collaboration in the scientific community is often encouraged by government agencies and the academic community, regarded as a way to boost scientific advancement.
The allegations were not outlandish. For years, the U.S. Justice Department had accused Chinese spy agencies of encouraging their nation's businesses to steal trade secrets from American corporations. Prosecutors here had wrung guilty pleas and convictions against scientists and businessmen who attempted to pilfer everything from fighter jet schematics to the details on how to make the pigment used to whiten the Oreo cookie stuffing.
In Xi's case, prosecutors said his work in the field of superconductors - which, when cooled to around 400 degrees below zero Fahrenheit, can help improve the efficiency of things from computer circuits to power grids - had just the type of potential applications coveted by a Chinese government in its race to surpass the United States in technology fields. They also can have military applications, as even Xi recognizes.
They accused him of sending schematics for a sophisticated piece of equipment known as a "pocket heater" to a colleague in China, despite a pledge he signed to keep its design a secret. They also accused him - in court papers that lacked many details - of offering to build a world-class laboratory in China in return for lucrative and prestigious appointments there.
While never formally accusing Xi of spying, U.S. prosecutors charged the professor with wire fraud, citing four emails he sent to Chinese contacts in 2010 about building the laboratory. If convicted, Xi faced up to 80 years in prison and a $1 million fine.
He pleaded not guilty, put his house up as collateral for bail, and - on the same day he was arrested - returned home.
Quickly, his once tranquil Penn Valley street became the epicenter of an international news story. Reporters shoved notes into the front door frame and under the windshield wipers of his car, as Xi and his family hid - curtains drawn - inside his home.
Long days turned into sleepless nights. Arduous decisions about whom to hire as a lawyer - and how much they could pay - dominated conversations. His wife, a physics professor at Pennsylvania State University, avoided campus. Colleagues whom Xi considered friends suddenly abandoned his side.
Xi claims that he was told not to return to Temple's campus after his arrest. (Temple spokesman Ray Betzner said Xi was never barred from campus; rather, the university made arrangements to allow him to supervise his research from home so he could better handle his case.) As interim chair of the physics department, Xi was placed on leave with pay.
Meanwhile, his lawyers contested prosecutors' claims in a behind-the-scenes battle of experts.
They argued that the government had fundamentally misunderstood the science behind his work: The schematics he designed were not for the pocket heater at all. Instead, the lawyers said, they were for a device of his own creation, one "totally different" from the pocket heater.
"There is no reason to imagine they are remotely similar," Xi said.
In August, Peter Zeidenberg, a Washington lawyer who had successfully represented several accused Chinese American scientists, delivered a presentation based on testimony from independent scientists, who concluded Xi had not shared the secret information, as prosecutors claimed.
The next month, the U.S. Attorney's Office issued a surprising announcement: It was dropping all charges. Prosecutors offered no explanation except to say "additional information" had come to their attention.
In the weeks that followed, life for Xi and his family inched back toward normal.
He returned to Temple, resumed classes, and ventured to a Washington event to publicly discuss the fallout from his case.
He was the latest poster child for a Chinese American community that accused the U.S. government of being too quick to accuse its members of international espionage. He joined three other Chinese American scientists in fields ranging from pharmaceuticals to hydrology who had been arrested in the previous two years, only to see prosecutors withdraw the charges.
Critics, including members of Congress, called it racial profiling - an allegation that the Justice Department has vehemently disputed - and called for an investigation.
Outside public view, Xi was reeling, he said: His reputation was mangled and he was buried under more than $200,000 in legal fees. He lost the chance to become permanent chair of Temple's physics department, a job he claims he had been promised just days before his arrest. (Temple disputes that plans for his chairmanship were ever concrete.)
Adding to the struggle, prosecutors had withdrawn charges "without prejudice" - meaning they had not ruled out charging Xi anew.
The window to reopen that specific case closed in March. In a statement last week, U.S. Attorney Zane D. Memeger, whose office filed the charges, avoided discussing Xi's case, but said charges are generally brought "based on evidence such as witness interviews, extensive document review, search warrants, and expert guidance."
And, he said, "On the rare occasion that if new information becomes available post-indictment which impacts the viability of a case or compromises the strength of evidence previously gathered, we recognize our responsibility to evaluate it."
Xi still remains haunted by the mystery of how it began.
"I see dangers all over the place," he said during a three-hour interview in his office last month. "I think I sound very annoyingly paranoid when I talk to my colleagues because I tell them, 'You better be careful, what you're doing is dangerous.' "
The case has also left an indelible impact on his family. "I don't think we can really see the world in the same way again," said his 23-year-old daughter, Joyce, a Yale University graduate with a chemistry degree.
It's that mentality and cases like Xi's - which are markedly on the upswing - that experts worry could have a chilling effect across the Chinese American and academic communities.
"That would certainly lead me, if I were doing research like that, to fear that kind of action by the government," said Henry Reichman, first vice president of the American Association of University Professors.
Xi is now returning to his research and instructed colleagues to do the same. He said he still considers America "the best country in the world."
But there's always room for improvement.
"If nothing good comes out of this," he said, "then we suffered so much for nothing."