At 04:15 (Moscow time) June 22, 1941, the German "Wehrmacht" attacked the Brest fortress with no warning. The attack started with an artillery barrage, including 600 mm mortars of the second battery of the Heavy Artillery Battalion 833 Nr. III ("Thor") and Nr. IV ("Odin"). The defenders were taken by surprise and initially failed to form a solid front. By 09:00 that day, the fortress was completely surrounded. The ensuing battle of Brest Fortress lasted for 7 days, during which about 2000 soldiers and officers defending the castle died; the attackers' casulties totaled to nearly 430 soldiers and officers.