At least 16 people were killed and more than 100 injured during riots on December 16 there, where tensions had risen after months of strikes and tent protests in the city center.
Officials said on January 25 that three police officers were charged with opening fire at rioters.
The new charge alleged that a fourth "senior" officer had beaten a suspected rioter to death.
When they initially announced the police cases, prosecutors also said three opposition politicians would face prosecution for allegedly inciting striking workers to violence. They include the head of the unregistered Algha (Forward) party, Vladimir Kozlov; the editor of the independent "Vzglyad" (Glance) newspaper, Igor Vinyavsky; and former presidential candidate Serik Saparghali.
Their arrests have been denounced by the Organization for Security and Cooperation in Europe (OSCE) and human rights groups.
based on agency and RFE/RL reports