Police searched the house of Zeljko Dilber, a comrade-in-arms and friend of Gotovina's, in the Zadar district of Borik on Thursday, seizing his mobile phone, computer and some other items.
The source, speaking on condition of anonymity, said that later in the day the police were also planning to interview Dilber and search his place of work in the Zadar office of the Jadransko Osiguranje insurance company and his boat.
The Zadar Police Department would neither confirm nor deny the information.
The Zagreb Police Department declined to say anything about the results of the alleged searches in the Zagreb area and the identity of the persons in question. The Interior Ministry said that a press release would be issued later in the day.
Dilber is one of the five Croatian citizens who have been banned entry to the European Union on suspicion that they are assisting fugitive war crimes suspects. The other four on the EU black list are Zeljko Bagic, Ljubo Cesic, Ante Maksan and Hrvoje Petrac.
Gotovina has been beyond the reach of the Croatian judiciary and the Hague tribunal since July 2001 when the tribunal indicted him for war crimes against Serb civilians committed during Operation Storm in the summer of 1995.