BEIRUT, Lebanon — Seven years ago, Islam Dabbas, an engineering student, was thrown in prison for protesting against the Syrian government. His mother visited him twice, paying bribes to do so, but then the permissions stopped. She heard nothing of her son’s fate ever since.

Until last week, when a relative filed for a government registration document and was shocked to see that it gave Mr. Dabbas’s date of death: Jan. 15, 2013.

“The news of his death devastated us, and we wish we had known then,” said his sister, Heba, who lives in exile in Egypt. “Since his arrest, we have lived days of hope and days of despair as uncertainty consumed our minds.”

In recent weeks, hundreds of Syrian families have suddenly learned that their missing relatives have been registered as dead by the government. Government officials have not commented publicly on the new information, said how many people it applied to, or explained how they died.