Enemy ground radio and radar interception units detected the approach of a bomber force and mobilised fighters across France, Belgium, Holland, Denmark and Germany. The fighters were directed towards the bombers until they could pick them up on their own airborne radar or by sight. Radar-equipped twin-engine fighters could intercept the bombers along their route. Once over the target, other free-ranging fighters (often single-engined ones, such as the Messerschmitt Bf 109) would join in. The Germans called these tactics Zahme Sau (tame boar) and Wilde Sau (wild boar) respectively.