In May losses were sustained. On 12 May Allied fighters shot down a Heinkel of 4./KG 55, whilst it was attacking railway targets North East of Reims, for the unit's first loss of the battle. The next day, 13 May, cost KG 55 ten machines, six from Stab. and 4./KG 55. On that day alone the unit's losses had exceeded that of the Polish Campaign. The losses suffered by KG 55 on 13 May were the worst of the war. A further seven machines were damaged and forced to land throughout the remainder of the fighting, although only two machines and crews were completely lost. The first of these, a 9./KG 55 Heinkel, was flown by "Unteroffizier" Horst Mahnert. Whilst returning from a mission to bomb airfields in the Lyon area on 2 June 1940 it strayed into Swiss airspace and was shot down near Ursins by "Capitaine" Hans Thurnheer in a Swiss Air Force Messerschmitt Bf 109.