The police have arrested four prisoners in connection with the incident being investigated as a murder.
The investigation is still in its early stage and the police will give a briefing on new developments Friday.
Tommi Saarinen, assistant director of Riihimäki Prison admitted that there is violence in the prison, but violence of such a serious extent is rare.
According to Saarinen, the prison is currently holding about 220 prisoners and is almost at its full capacity.
Saarinen said the jail is meant for long-term prisoners held for serious crimes of violent nature and also for drug-related crimes.
