Complex sentence: In the Bonin case, although the killer was in custody, the DA's office was having trouble building a case, until one day Bonin received a letter from the mother of one of his victims, begging him to tell her what happened to her son.
Split into simpler sentences: In the Bonin case, although the killer was in custody, the DA's office was having trouble building a case. Until one day Bonin received a letter from the mother one of his victims, begging him to tell her what happened to her son.