Original: This gas chamber operated from 1941 to 1942, during which time some 60,000 people were killed therein; it was then converted into an air-raid shelter for the use of the SS.
Simplified: They used this from 1941 to 1942, and killed about 60,000 people by sending them to the gas chamber. After this, they made this as an air-raid shelter for the use of SS.