China has detained an American seismologist tracking nuclear tests for nearly two years on espionage charges, his family says.

Chen Youlin, 54, was arrested in November 2024 during a trip to Beijing to visit his family, according to the hostage advocacy group Global Reach. The family decided to speak out after they saw no signs of Beijing releasing Chen.

His wife Rong Yufang, also a seismologist, said Chen worked closely with Chinese colleagues and that the accusations are “both erroneous and inconsistent with the public and collaborative nature of the work he has done.”

His published work focuses on North Korea, a close friend of China and long sanctioned for its nuclear weapons program and underground testing.

It is unclear if and how Chen’s work affected Beijing’s nuclear program. US intelligence suggests China is developing a new arsenal and has conducted secret tests, which Beijing denies.

When asked about the case at a daily news briefing on Tuesday, China’s Foreign Ministry said its “judicial authorities handle the cases in accordance with the law.”

“There is no such thing as so-called unjust detention,” ministry spokesman Lin Jian said. In China, convictions for espionage can carry life in prison or death.

Chen is currently the only US citizen designated as “wrongfully detained.”

“I have not been able to speak to my husband for over 600 days and I am concerned for his health and well-being,” Rong said in a statement through Global Reach.

In an interview with Reuters, she said Chinese authorities questioned her husband more than 100 times about his work and did not allow him to see a lawyer during the first 13 months of his detention.

Born in China, Chen became a US citizen in 2011. He lives in Boston, Massachusetts.

He specialized in using seismological data to identify nuclear tests and undertook several projects funded by the United States government. Rong said his work with his colleagues in China was always done in a “transparent” manner.

“He is making precisely the kind of people-to-people engagement that the Chinese government says it wants,” he added.

Chen’s work included a study conducted in December 2020 that analyzed seismic data recorded in Asia, including China, to improve methods for tracking nuclear tests and estimating yield.

According to Global Reach, there are “suspicions within the US government that Chen’s arrest was motivated by China’s conduct of nuclear testing in violation of the Comprehensive Nuclear Test Ban Treaty.”

The group said Chen’s experience would give China “the opportunity to learn as much as possible about the United States’ seismic detection methodologies so they can establish countermeasures that would allow them to circumvent the treaty.”

The treaty seeks to ban all testing of nuclear explosions on Earth, but several “nuclear capable” states have not yet ratified it. Among them are the United States and China, which have established voluntary moratoriums against explosive nuclear tests.

In June 2020, during Donald Trump’s first presidency, his administration accused Beijing of conducting a covert underground nuclear test at the Lop Nur facility in the country’s northwest. China dismissed the claims as baseless and politically motivated.

The Foley Foundation, another U.S.-based hostage advocacy group, said Chen’s health is a concern, noting that he suffers from diabetes, high blood pressure and high cholesterol.

“He needs reliable access to treatment and care that is not available while he is wrongfully incarcerated,” the group said.

U.S. Sen. Edward Markey, D-Mass., said Beijing’s “treatment of Chen has undermined [its] association [with the US] and may discourage other academics from collaborating with their colleagues in China.”

“It is my hope that greater attention to his unjust detention will force the Chinese government to do the right thing and release Chen,” he wrote in a statement released Tuesday.

Details about Chen’s detention came a month after China confirmed it had arrested another American academic, Min Zin, director of a think tank focused on Myanmar.

Beijing has accused Min Zin of spying and endangering Chinese national security.