Russian history is just brutal, particularly during the Soviet era. A meaningful portion of its citizens were deemed criminal and the definition of criminal changed with time. All sorts of things got you dead or imprisoned: your class, your region, your town, ethnicity, religion, friends, family, profession and everything else.

There will be loads of good books about it, but Simon Sebag Montefiore has done a few - Young Stalin and The Court of the Red Czar come to mind immediately.

They weren't considered criminals, they were sent to detention for clearing because otherwise it would've been easy for spies to get into army ranks. Of course, it sounds straight-forward in theory, but during war times the conditions in the detention were awful, and the whole Soviet system of gulags was very demanding of labor, so there weren't many incentives to get cleared.