HANOI, Vietnam — Do Thi Mai said she was shocked to learn that her 17-year-old son, Do Dang Du, had fallen into a coma in prison a few weeks after he was arrested, accused by the police of stealing about $90.

The police initially said that Mr. Du’s severe head and leg wounds had been caused by falls in the bathroom, according to a family lawyer. “He was unconscious, so I couldn’t ask him,” Ms. Mai said.

Mr. Du died in the hospital a few days later, in October 2015, and members of his family told an interviewer that they believed he had been tortured in custody. The next month, two of their lawyers were assaulted outside the family home by what the lawyers said were eight masked men.

Nearly two years later, Ms. Mai is still searching for closure. “Two months before he died, he was healthy,” she said during an emotional interview at home on the rural fringe of Vietnam’s capital, Hanoi. “How did he turn out like this?”