The United Nations committee, the Working Group on Arbitrary Detention, is under the Human Rights Council. The Dui Hua Foundation, a group based in San Francisco that seeks to secure the release of political prisoners in China, said on Tuesday that this was the first time in the 25-year history of the working group that it had judged that an American citizen had been deprived of rights and arbitrarily detained by the Chinese government.

Ms. Phan-Gillis, a Vietnamese-American business consultant from Houston, was secretly detained in March 2015 by officers from the Ministry of State Security, which oversees espionage and counterespionage. She had been traveling in southern China with a group of businesspeople and officials from Houston.

In September 2015, her husband, Jeff Gillis, announced that he had heard that China had formally detained her and that the case was no longer secret. He said he had received the news just two days before President Xi Jinping of China was to arrive in Seattle for the start of a trip that would culminate in a state visit to Washington.

The United Nations working group said that the Chinese government had told it in April that Ms. Phan-Gillis was being accused of trying to steal state secrets and aiding an outside party in gathering national intelligence. She was formally arrested in October 2015, the group said.

The working group came up with its opinion on the detention during a meeting in April and released the findings last week. The written opinion said the group had learned that state security officers had stopped Ms. Phan-Gillis on March 19, 2015, at the border crossing between Zhuhai, in mainland China, and Macau, a Chinese special administrative region. She was held for six months in a secret location, commonly known as a black jail, then transferred to a detention center in Nanning, the provincial capital of Guangxi, once officers had decided to formalize her detention.