One of Mr. Shapovalenko’s cellmates, a man in his 50s named Ihor, died from the vicious beatings he had received, he said. Ihor was interrogated for three or four days, and after they returned him to the cell, the Russian guards ordered him to write a statement and kept rousing him to prevent him from sleeping. On the fourth day they let him sleep, but by then it was too late and he died that night.

“They never read his testimony,” Mr. Shapovalenko said. “All of us thought we would end up like that.”

Another man, Serhii Ruban, 42, a sales consultant, also died in the detention center, prosecutors have established. His mother, Nina Ruban, 70, said she last saw him alive when he was arrested on June 12. Six days later, she was told at the army headquarters that her only son was dead.

Two witnesses saw him being heavily beaten in the corridor and inside their cell, prosecutors said, and a third witness moved his body to the morgue. Investigators found his body among the remains in a mass grave, and in February, his mother identified him by a tattoo on his knuckles. He had multiple rib fractures, leaving her in no doubt that he was beaten to death.

“He was all broken,” she said, weeping.

Oleksandr Chubko and Dyma Shapoval contributed reporting.