Police spokeswoman Aleksandra Ljuba confirmed that the investigation was under way, but would not reveal any details.
Jutarnji List daily today reported that Zagorec's friends Josip Ferak, Roman Binder and Krunoslav Trajkovic were interviewed by the police on Saturday.
Trajkovic was reportedly stopped at a border crossing while travelling to Vienna to visit Zagorec. The police reportedly found in his car classified documents, after which the flats and business premises of Zagorec's three friends were searched.
The daily speculates that the documents are connected with the ongoing proceedings against Zagorec, who is suspected of misappropriating USD 5 million intended for Croatia's defence during the 1991-1995 war. He served from 1993 to 2000 as assistant defence minister in charge of arms procurement.
Zagorec has been living in Austria for the past seven years. He was arrested in Vienna in March this year on an international warrant issued by a Croatian court. After a hearing at a Vienna court, he was released on bail.
On March 21, the Justice Ministry sent Austrian authorities a request for his extradition, which Zagorec objects to.