Complex sentence: Cement takes hours to harden and, until 2016, there was never a documented case--although crime historian Thomas Reppetto said there have probably been many real-life examples that have never been found.
Split into simpler sentences: Cement takes hours to harden and, until 2016, there was never a documented case. Crime historian Thomas Reppetto said there have probably been many real-life examples that have never been found.