President Vladimir Putin arrived in Kemerovo earlier and blamed "criminal negligence" for Sunday's blaze.
Relatives say dozens of people - including children - are still missing.
Investigators say the fire alarm was switched off and exits blocked. Four people have been charged.
Another detained suspect - a guard from a private security firm - will be charged shortly, Russia's Investigative Committee says.
A national day of mourning will be observed on Wednesday.
Source:
BBC