Beijing — An American scientist who has studied underground nuclear tests has been detained in China for more than 18 months on spying charges, according to his supporters and a US lawmaker.

Youlin Chen, a seismologist, has been “wrongfully detained” since November 2024, US Senator Edward Markey said in a statement Tuesday. President Donald Trump raised Chen’s detention and asked for his freedom during a meeting with Chinese leader Xi Jinping in Beijing in May.

Chen, who was living in Boston and has a college‑age son, is the only American currently held in China designated as wrongfully detained, the non‑profit Global Reach said. The case adds friction between the US and China as they try to stabilize ties, ahead of Xi’s expected visit to the US later this fall.

Global Reach said suspicions that Chen’s detention is linked to China’s recent expansion of its nuclear capabilities, including an alleged underground nuclear test in 2020. Beijing denies this test. China and the US have both signed, but not ratified, the Comprehensive Test Ban Treaty (CT BT). Chen’s work focuses on seismological data to improve methods to identify and monitor nuclear tests, including North Korea.

Chen was detained at the airport after visiting his parents in Beijing, according to Kieran Ramsey, chief investigative officer at Global Reach. He was charged with espionage on May 1, 2025, but has not yet stood trial.