One day after his lecture at the China University of Geosciences in Beijing, Dr. Chen went to the airport to catch his flight. Then, unexpectedly, he messaged his wife that he had changed his plans. Ms. Rong sent him a message but got no reply.

Chinese investigators seized Dr. Chen’s phone, iPad, laptop and cameras, and appeared to have gained access to his online accounts, including his email, and rifled through its contents, according to Senator Ed Markey of Massachusetts, citing details provided by Ms. Rong. Sen. Markey has worked to secure Dr. Chen’s release. In a letter to the Secretary of State Marco Rubio, Mr. Markey said that the location data from Dr. Chen’s phone appeared to show that it was at an office of the Beijing state security bureau. Sen. Markey’s office shared a copy of the letter with The New York Times.

Dr. Chen could not be reached. He was formally arrested on May 1, according to Mr. Lebson.

American consular officials have been allowed to visit Dr. Chen in the presence of Chinese officials. Ms. Rong said Dr. Chen told them he has lost as much as 40 pounds while in detention, and has not been able to get the medicines he usually takes for chronic illnesses, including diabetes and high blood pressure. It was unclear if or when Dr. Chen might stand trial.

In March, Mr. Rubio officially designated Dr. Chen as “wrongfully detained,” a step that raises the urgency of a case under legislation that took force in 2020, according to Elizabeth Richards, the director of hostage advocacy and research at the James W. Foley Legacy Foundation, which has also campaigned for Dr. Chen.

“That designation is significant, because it is the U.S. government saying this person is innocent and being held for political reasons,” she said.

For now, the Chinese government is not giving ground. In 2015, China released Xue Feng, an American geologist who was born in China, after he had been detained and imprisoned for more than seven years for “illegally procuring state secrets.” His release and return to the United States came after years of lobbying by Washington.

Edward Wong contributed reporting from Washington.