Complex sentence: Investigators and the public also focused on McStay's business partner, Chase Merritt, who was the last known person to have had contact with Joseph McStay, and was the first to notice his disappearance.
Split into simpler sentences: Investigators and the public also focused on McStay's business partner, Chase Merritt. Merritt was the last known person to be in contact with Joseph McStay, and the first to notice his disappearance.