Complex sentence: But in December 1944, he was saved by American Jews who had paid a ransom to have a number of Jews, including him and his parents, released from the concentration camp, where 100,000 Jews had already been killed.
Split into simpler sentences: But in December 1944, he was saved by American Jews. They paid a ransom to have a number of Jews including him and his parents released from the concentration camp, at which 100,000 Jews had already been killed.