Zagorec, a retired general, is suspected of abuse of office, namely that in 2000 he took from a Defence Ministry safe diamonds he had received from a German arms dealer in 1993 as collateral for five million US dollars from the ministry intended for the purchase of weapons for Croatia's defence.
Petrac, a businessman, and Zagorec's former secretary Siprak were the first to go public with the jewels story at Petrac's retrial for the kidnapping of Zagorec's son. They were interviewed separately by investigating judge Erna Drazancic.
What the two said in the interviews is unknown because Drazancic had granted the prosecution's request that the interviews and all other investigative proceedings be declared secret, court spokesman Kresimir Devcic told the press.
None of Zagorec's attorneys appeared at the hearing. Drazancic had turned down the request by one of them, Kresimir Krsnik, that the hearing be postponed due to his illness.
Another three witnesses will be heard tomorrow and two more on Thursday, Devcic said, declining to reveal their names until after they were interviewed.
The media said that Miroslav Tudjman, a former intelligence chief, and German arms dealer Josef Rothaichner were expected to be questioned, the latter by videolink.
An investigation into Zagorec was launched on March 9. Four days later, he was arrested on an international warrant in Vienna, where he has been living for the past seven years. He was released from custody after posting EUR1 million bail.
On March 21, the Croatian Justice Ministry sent the Austrian authorities a request for Zagorec's extradition, to which he objects, claiming the Croatian investigation is politically motivated.