SEOUL, South Korea — Phillip Clay was adopted at 8 into an American family in Philadelphia.

Twenty-nine years later, in 2012, after numerous arrests and a struggle with drug addiction, he was deported back to his birth country, South Korea. He could not speak the local language, did not know a single person and did not receive appropriate care for mental health problems, which included bipolar disorder and alcohol and substance abuse.

On May 21, Mr. Clay ended his life, jumping from the 14th floor of an apartment building north of Seoul. He was 42.

To advocates of the rights of international adoptees, the suicide was a wrenching reminder of a problem the United States urgently needed to address: adoptees from abroad who never obtained American citizenship. The Adoptee Rights Campaign, an advocacy group, estimates that 35,000 adult adoptees in the United States may lack citizenship, which was not granted automatically in the adoption process before 2000.

Mr. Clay is believed to be just one of dozens of people, legally adopted as children into American families, who either have been deported to the birth countries they left decades ago or face deportation after being convicted of crimes as adults. Some did not even know they were not American citizens until they were ordered to leave.