Complex sentence: Reimer was first investigated by American authorities in 1980 in connection with the John Demjanjuk case, but it was not until 1992, after the fall of the Soviet Union, did investigators make substantial progress.
Split into simpler sentences: Reimer was first investigated by American authorities in 1980 in connection with the John Demjanjuk case, but little progress was made during this initial investigation. Not until 1992, after the fall of the Soviet Union, did investigators make substantial progress.