Since an American therapist from Texas disappeared while visiting Syria in 2017, his family has navigated years of milestones in uncertainty: his 41st wedding anniversary, the births of four grandchildren, his wife’s cancer diagnosis.

Earlier this month, at a hotel in Washington, national security officials told the family that highly credible, classified information indicated the therapist, Majd Kamalmaz, had died in captivity, held in one of the world’s most notorious prison systems.

Mr. Kamalmaz’s relatives intend to grieve soon, finally, at a mosque and at the family’s home in Grand Prairie, Texas. They had hoped for a different outcome, but now that they have accepted the news, his daughters said they would fight to hold Syria accountable for their father’s detainment and death. The family plans to sue the Syrian government for damages and to seek justice for others still in detention.

“They literally kidnapped him and disappeared him,” said Mr. Kamalmaz’s older daughter, Ula Kamalmaz. “We’ve heard nothing from them. That’s unacceptable.”

Mr. Kamalmaz is one of several Americans who have disappeared in Syria. Austin Tice, a freelance journalist covering the country’s civil war, was abducted outside Damascus in 2012. Layla Shweikani, 26, an American aid worker, was arrested, accused of terrorism and executed. The Justice Department is investigating her 2016 death as a war crime carried out by Syrian intelligence officials.