The bomb blew windows out of the building housing three international organisations, destroyed one OSCE vehicle and damaged another. But S-For sources said they thought it was an "isolated incident".
Zvornik is on the other side of Bosnia from Prijedor, where British SAS troops killed an indicted war crimes suspect - former police chief Simo Drljaca - on Thursday. Another suspect, Milan Kovacevic, wanted for atrocities at the Omarska concentration camp, was taken alive, and is now in The Hague awaiting trial at the international war crimes tribunal.
"We do not see a widespread, orchestrated security problem. We see isolated incidents," said Duncan Bullivant, spokesman for Carlos Westendorp, the High Representative to Bosnia. Until Sunday night, the Bosnian Serbs were restrained in their reaction to S-For's first seizure of war crimes suspects. And although Bosnian Serb leaders, including the elected president, Biljana Plavsic, condemned the arrests, they, too, appealed for calm.
Drljaca was buried on Sunday, and the emotion generated may have triggered the bomb attack.
Ms Plavsic, who was elected last September, has come under increasing criticism from opponents in the Bosnian Serb republic. Yesterday she rejected attempts by nationalist rival, Momcilo Krajisnik, to step down. She said she would continue her crusade against corruption and to "establish a state ruled by law".
The OSCE supervised the elections and may have been an appropriate, as well as soft target for Ms Plavsic's opponents.
Until Thursday, S-For had avoided seizing war crimes suspects for fear of provoking armed resistance from the local parties. But the presence of wanted war criminals has threatened the elected authorities.
S-For believes the swoop on Thursday was worth the risk, even though it has alerted the 67 publicly indicted war crimes suspects still at large, and also about six who are subject to "sealed indictments".
The big fish - Radovan Karadzic and Ratko Mladic - are now being doubly cautious. Mr Karadzic is believed to be at home, near Pale, in the French sector. Mr Mladic is reported to have taken a holiday in Montenegro, but may now be in the Bosnian Serb army headquarters at Han Pjesac.Reuse content