> How do you think the wrongly arrested people confessed to the murders in the first place?
As in that example with Kravchenko, the same way false confessions occur in every country, pressure in some form from the police who want to convict their prime suspect / clear up a case.
It's indefensible but it's nothing unique to the Soviet Union and it's not uncommon.
"At his trial, Kravchenko retracted his confession and maintained his innocence, stating his confession had been obtained under extreme duress."