It wasn’t the call Oleg Buryak expected.

He was hoping to hear that his 16-year-old son, Vlad, had safely escaped the Ukrainian city of Melitopol, where Moscow’s forces were quickly closing in. Instead, it was a Russian military man on the other end of the line.

They had taken his son, the soldier said, and he was being kept in an undisclosed location.

Almost overnight, Buryak, head of the Zaporizhzhia Regional Military Administration, was thrust into a frantic, detective-like pursuit, scrambling for clues, trying to figure out where Russian soldiers were holding his son, and how to get him back.

Soon, Vlad found a guard who allowed him to make occasional calls. The teenage boy was growing desperate, his father said. At home, Vlad loved computer games. In his cell, he was surrounded by the constant, terrible sound of other prisoners being tortured.

“What are you doing to get me out of here?” Vlad asked his father.

For nearly four months, the world has watched in horror as Russian forces flattened Ukrainian cities, with images of slaughtered civilians in Bucha and Mariupol attracting international outrage and prompting Western powers to increase their military aid. But all the while a less visible phenomenon was taking place in homes, at checkpoints, during street protests: Russian soldiers were detaining and abducting hundreds — perhaps thousands — of civilians.