During the initial period of Birkenau planning, it was the biggest brick object, and the watchtower was the tallest building. Construction was completed in 1942,but it was enlarged in 1944 as room was needed for more guards. In 1944, due to air raid danger it was painted in camouflage pattern. There was a siren on top of the tower to warn of danger or the escape of a prisoner. Beginning in 1942, Birkenau was the setting of the most massive murder campaign in history. Construction of the camp began in 1941, initially planned as a camp for Soviet prisoners of war, but it ended up being a concentration camp for persons of various ethnic backgrounds. It was initially to hold 97,000 persons with plans revised up to 200,000. By 1944, there were 90,000 prisoners, mostly Jews.