Beijing —
An American scientist who has studied underground nuclear tests has been detained in China for more than 18 months on suspicion of espionage, according to his supporters and U.S. lawmakers.
Seismologist Yulin Chen has been “unlawfully detained” since November 2024, U.S. Sen. Edward Markey said in a statement Tuesday.
President Donald Trump raised Chen’s detention and called for her freedom during a meeting with Chinese leader Xi Jinping in Beijing in May, according to Global Reach, a U.S. nonprofit that is working with Chen’s family on the case.
Chen, who lives in Boston and has a son in college, is the only American currently being illegally detained in China, the nonprofit said.
The incident adds another point of friction between the United States and China, which are seeking to stabilize relations ahead of President Xi’s visit to the United States later this fall. The revelation came weeks after China confirmed the arrest of another American academic, Ming Jing, on “suspects of espionage and endangering China’s national security.”
Global Reach said Chen’s detention is suspected to be related to China’s recent expansion of its nuclear capabilities, including conducting an underground nuclear test in 2020, which the Chinese government denies.
China and the United States have both signed the Comprehensive Nuclear-Test-Ban Treaty (CTBT), but have not ratified it. This is an international agreement that prohibits “all nuclear weapons test explosions and other nuclear explosions.”
Chen’s research focuses on using seismic data to improve ways to identify and monitor nuclear tests. It also includes an investigation into North Korea’s underground nuclear tests.
Chen’s research is funded by the U.S. Department of State and the U.S. Air Force Research Laboratory.
In December 2020, he authored a technical report that used regional seismic data recorded across Asia, including data from observation stations in China, to improve methods for monitoring nuclear tests and estimating yields, according to Global Reach.
A subsequent 2024 study he authored was also funded by the U.S. Air Force Research Laboratory and the U.S. Department of State and “further strengthened Chen’s expertise in seismic monitoring and underground nuclear test detection,” the research group said.
Global Reach lead investigator Kieran Ramsey said Chen was detained at the airport after visiting his parents in Beijing.
“China has been accused by the State Department of violating the (CTBT),” Ramsey said. “And at the same time, they are detaining American experts who could pinpoint it.”
“Unfortunately, this incident is yet another example of China attempting to use hostage diplomacy as part of its great power competition with the United States, and unfortunately Dr. Chen is currently the only example officially designated by the U.S. government,” he added.
Earlier this month, the pastor who founded one of China’s most prominent underground churches was released and reunited with his family in the United States after being detained in a crackdown in China last year. His release came after President Trump raised the issue with Xi Jinping during a visit to Beijing in May.
A State Department spokesperson told CNN that the United States has raised Chen’s case directly with Chinese authorities and asked for his immediate release.
Asked by CNN if Trump had raised the issue during his meeting with Xi, a White House official said: “President Trump has made it clear that he wants all Americans detained overseas to return home.”
When asked about the incident at a regular press conference on Tuesday, China’s Foreign Ministry denied that Chen had been “unlawfully detained” and said judicial authorities were handling the case in accordance with the law.
Chen was indicted on May 1, 2025 on charges of espionage, but has not yet been tried. According to Global Reach, Trump’s family has decided to speak out about his detention now because their requests have gone unheeded.
Chen’s wife, Yufan Long, said she had not spoken to her husband in more than 600 days and was worried about his health condition.
“Mr. Yulin never held a U.S. government security clearance, and to suggest he was involved in espionage is false and inconsistent with the public and collaborative nature of the work he did,” she said in a statement provided by Global Reach.
She noted that her husband “works transparently with his Chinese colleagues on scientific cooperation,” adding: “He has exactly the kind of people-to-people exchanges that the Chinese government wants.”
Long told Reuters in an interview that U.S. embassy officials have visited Chen several times, but Chinese officials are always present and they are unable to speak freely. She hired a Chinese lawyer, but he was only allowed to meet Cheng after the scientist had been detained for more than 13 months.
Chinese authorities have interrogated her husband more than 100 times about his work on seismic records of North Korea’s nuclear tests, she said.
