The Germans investigated, but concluded Enigma had not been breached by either seizures or brute force cryptanalysis. Nevertheless, the Germans took some steps to make Enigma more secure. Grid locations (an encoded latitude and longitude) were further disguised using digraph tables and a numeric offset. The U-boats were given their own network, Triton, to minimize the chance of a cryptanalytic attack. In August 1941, the British captured . The Germans concluded the crew would have destroyed the important documents, so the cipher was safe. Even if the British had captured the materials intact and could read Enigma, the British would lose that ability when the keys changed on 1 November. Although Germany realized that convoys were avoiding its wolfpacks, it did not attribute that ability to reading Enigma traffic. Instead, Dönitz thought that Britain was using radar and direction finding. The Kriegsmarine continued to increase the number of networks to avoid superimposition attacks on Enigma. At the beginning of 1943, the Kriegsmarine had 13 networks. The Kriegsmarine also improved the Enigma. On 1 February 1942, it started using the four-rotor Enigma. The improved security meant that convoys no longer had as much information about the whereabouts of wolfpacks, and were therefore less able to avoid areas where they would be attacked. The increased success of wolfpack attacks following the strengthening of the encryption might have given the Germans a clue that the previous Enigma codes had been broken. However, that recognition did not happen because other things changed at the same time, the United States had entered the war and Dönitz had sent U-boats to raid the US East Coast where there were many easy targets. In early 1943, Dönitz was worried that the Allies were reading Enigma. Germany's own cryptanalysis of Allied communications showed surprising accuracy in its estimates of wolfpack sizes. It was concluded, however, that Allied direction finding was the source. The Germans also recovered a cavity magnetron, used to generate radar waves, from a downed British bomber. The conclusion was that the Enigma was secure. The Germans were still suspicious, so each submarine got its own key net in June 1944. By 1945, almost all German Enigma traffic (Wehrmacht military; comprising the Heer, Kriegsmarine and Luftwaffe; and German intelligence and security services like the Abwehr, SD, etc.) could be decrypted within a day or two, yet the Germans remained confident of its security. They openly discussed