The Croat is 33 years old and lives around Zagreb.
Slovene commercial TV station POP-TV said on Monday the police last week seized 37 kg of explosives in an operation near Celje and Ljubljana, including a significant amount of Goma-2 ECO, which al-Qaida used for the attack in Madrid.
Also seized were six guns and 30 electric detonators, said POP-TV, quoting Celje crime police chief Robert Mravljak.