Declining to disclose the suspect's identity, spokesman Zlatko Mehun told Hina that the man had had no criminal records so far.
More information will be offered at a news conference later in the day.
Two people were shot dead in Saturday's armed raid on the Fina office in Djurdjevac. The incident took place around 9am when a man entered the office and started shooting, instantly killing a client who happened to be in the office and wounding a security guard, who later died in a hospital. It was not immediately clear how much money was stolen. The police launched an investigation and mounted a massive manhunt, sealing off the town and interviewing dozens of people.