A pressure-activated anti-personnel mine planted under Tadic's car exploded in the centre of Zenica, Bosnia and Herzegovina, last Monday, causing no injuries, according to the Bosnian prosecutorial authorities.
The incident happened while the car was parked in a parking space outside the building housing the cantonal institutions. The explosive device went off when Tadic entered his car and tried to start it after attending the local assembly's session. The blast caused minor damage to the vehicle, according to findings of a police on-site investigation.
The Bosnian prosecutorial authorities have said that the incident is most likely politically motivated.
The HDZ BiH condemned the incident, labelling it as an act of terrorism. The party said that recently its officials had been receiving threats in areas predominantly populated by Bosnian Muslims (Bosniaks).
Of the four arrested people, three were arrested in Zenica and one in Zavidovici.
The Office of the State Prosecutor said in a statement that they were suspected of a terrorist act aimed at intimidating the population and undermining the political and social structures of the Federation of Bosnia and Herzegovina.