An explosive device has been planted at a courthouse in the French city of Annecy.
The explosion blew out windowpanes and badly damaged the building but caused no injuries.
The blast on Monday night left a 20-inch crater and shattered windows in nearby buildings.
The damage to the courthouse was especially severe because of the number of windows in the building.
Prosecutor Denis Robert-Charrerau says authorities believed an explosive device triggered the blast but have not yet pinpointed what type of explosive had been used.
"There's a lot of damage, especially because of the effect of the blast on this building made of glass," Mr Robert-Charrerau said.