The incident occurred on Wednesday in Vukovar's Borovo Naselje district when an explosive device was thrown at the premises of the local HSP branch office.
"The political and inter-ethnic atmosphere in Vukovar has improved considerably over the last couple of years," Semneby said in a statement issued on Friday. "It is important not to allow the positive changes to be jeopardised by senseless acts of violence or rash reactions to them."
Semneby said he was concerned by statements made by some politicians recalling war-time divisions. He also commended the Vukovar county police for ensuring that the suspects were promptly brought into custody.