Complex sentence: He never revealed who betrayed those in the annex, and indeed, was probably never questioned about it, as the subject of who, specifically, had betrayed the eight people in the annex did not become a point of interest until long after Dettmann's suicide.
Split into simpler sentences: He never revealed who betrayed those in the annex, and indeed, was probably never questioned about it. That query did not become a point of interest until long after Dettmann's suicide.