 The Germans attacked Leningrad in August 1941 ; in the following three " black months " of 1941 , 400 @,@ 000 residents of the city worked to build the city 's fortifications as fighting continued , while 160 @,@ 000 others joined the ranks of the Red Army . On 7 September , the German 20th Motorized Division seized Shlisselburg , cutting off all land routes to Leningrad . The Germans severed the railroads to Moscow and captured the railroad to Murmansk with Finnish assistance to inaugurate the start of a siege that would last for over two years . 
