An investigation into the incident established that on May 31 four inmates attacked prison employees during a walk in the prison, injuring five of them. Guards beat the men using rubber truncheons and handcuffs, and they died several hours later.

"An investigation into the disorder at the Kopeysk colony is underway. A court extended the custody of six prison guards until April 30, 2009," the source said.

Multiple charges, including the abuse of office have been laid against 16 prison guards. Six of them are kept in a pre-trial detention facility and 10 have signed written pledges not to flee.

The investigation is expected to be completed after May 2009.