Weapons [object-を] held hijackers [topic-は] passengers [object-を] horrified made.
慄然とさせた uses the causative of する with the mimetic 慄然と (in a state of horror). 乗っ取り犯 (hijacker) is a compound: 乗っ取り (hijacking) + 犯 (criminal). 武器を持った is a relative clause modifying 乗っ取り犯たち.