Complex sentence: This account has since been discredited as lacking foundation in documentary evidence, and Jaster now appears to have been rearrested by the Gestapo in Warsaw on 12 July 1943 and to have perished sometime between July and September of that year.
Split into simpler sentences: This account has since been discredited as lacking foundation in documentary evidence. What now appears to be reasonably certain is that Jaster was rearrested by the Gestapo in Warsaw on 12 July 1943, and that perished sometime between July and September of that year.