Boskoski is suspected of having ordered in 2002, in his capacity as Macedonia's interior minister, the killing of seven economic immigrants from Pakistan and India who he thought were terrorists.
Explaining why detention was extended, Stanic mentioned the flight risk and the circumstances under which the crime was committed.
Boskoski, who also has Croatian citizenship, was arrested in Croatia on August 31.
His lawyers said they would appeal the extended detention with the Supreme Court.