“We received information that she is doing O.K.,” according to Ms. Petersen’s GoFundMe page. “She wakes up when told, she goes to sleep when told. She spends her day in a jail cell or walking in a circle counting steps.”

It is unclear when or where trials for Mr. Harlan and Ms. Petersen will be held. A spokesman for the United States Embassy in Beijing said United States officials were aware of the detentions and were “monitoring the situation.”

The police in Zhenjiang did not respond to multiple requests for comment on Thursday.

The detentions come as tensions grow between China and the United States over trade and other issues, but Mr. Geng, the Chinese Foreign Ministry spokesman, said on Thursday that he did not see a “specific correlation” between the two.

But recent detentions of Americans have raised concerns. In June, a Koch Industries executive was held in southern China and interrogated for several days before being allowed to leave the country. Last month, a FedEx pilot was detained on charges of weapons smuggling. Though he was quickly released on bail, he remains under investigation and has not been allowed to leave the country.

In January, the State Department issued a travel advisory for China, warning Americans, particularly those with dual Chinese-American citizenship, that they may be prevented from leaving China if they go there.

The large network of English teachers in China, long a mainstay of the expatriate scene, appears to have come under particular scrutiny. For years, many education companies operating in China were known to have dabbled in a legal gray area, as the authorities often looked the other way.

Reports were widespread of foreign teachers working in China without proper visas. According to official estimates, while China had around 400,000 foreign citizens working in its education industry in 2017, only one-third had a valid work permit. Some schools, in turn, exploited that vulnerability, in some instances by withholding wages.