The blast occurred after the explosive device had been placed in a fire, a spokesman for the south prosecutor's office, Marko Uibu, said. "An investigation has to establish exactly what type of explosive device was involved," he added.
Uibu said the security police has launched an inquiry, basing the criminal proceeding on the Penal Code article dealing with causing an explosion by using an explosive device or explosive substance. Conviction carries a prison sentence of up to 10 years.
The explosion in which four 15-year-old students of Tartu's Russian gymnasium were wounded occurred around 6 p.m.
The Tartu University hospital to which the injured teenagers were taken said two are in a serious condition and one in a critical condition. The fourth casualty was discharged after being given treatment.