U.S. legal residents may have disappeared into the sprawling network of detention camps in China’s northwestern Xinjiang Province, Sam Brownback, U.S. Ambassador at Large for International Religious Freedom, said at a press briefingThursday.

Brownback told reporters in Washington D.C. that he had received an unconfirmed report that a California man’s father, a U.S. legal resident, disappeared after visiting Xinjiang several months ago.

“I received [an] e-mail from a gentleman in the United States whose father is still in Xinjiang who hasn’t – he’s not been able to reach him for months, 75-year-old man, doesn’t know whether – where he is and whether he’s still alive.” Brownback said.

CNN quoted State Department sources saying “a few” U.S. residents are being detained.

Read More: Chinese Official Compares Xinjiang Detention Centers to ‘Boarding School’ and Says They May Be Phased Out

China is facing international condemnation over its practice of arbitrarily imprisoning what the State Department estimates is between 800,000 to up to 2 million Turkic Muslims, including Uighurs, ethnic Kazakhs, Kyrgyz and others in “re-education camps.” U.S. officials have repeatedly denounced the practice, which China has defended as an effective means to de-radicalize the borderland and prevent terrorism. Chinese officials have also tried to pass off the camps as a form of “vocational training.”

Secretary of State Mike Pompeo held private meetings with several Uighurs in Washington D.C. earlier this week, where he made renewed calls for China to close down its “abhorrent” detention centers.

Read More: Turkey Demands China Close ‘Concentration Camps’ Holding Muslim Uighurs

“I join the Secretary’s call that China must end these counterproductive policies, release all arbitrarily detained, and end its repression that is taking place,” Brownback said. “It’s being raised in multiple forums by the United States, and consistently,” he added.

Write to Amy Gunia at amy.gunia@time.com.