Because Sheriff P. Bartlett made a statement on September 12th, two days after the body had been found in Kinderhook, and said that though "no one [had] been arrested ... the public was not at risk", that there were "no suspects at large", and that the department felt confident that "that there [wasn't] an individual running around who would cause harm to the public", I have the following two guesses:
1. Upon receiving the tip (presumably from Molly), and then discovering the body, the cause of death was clearly an overdose to police. I'm not sure, how, exactly, the cause of death could SO obviously be an overdose unless there were a needle sticking out of someone's arm, but this is one way a sheriff could believe that there is absolutely no threat to the public from an "individual running around". If the COD is an overdose, we will know that when the autopsy results come back. I believe that generally takes approximately 2 weeks, but please correct me if I'm wrong.
2. The second guess I have is that the suspect is in custody, though has perhaps not been arrested or charged in the murder/death of Schmidt. Perhaps he was taken into custody for a different crime, and the department has been gathering information/evidence in order to charge him with this one. There was a Facebook post made by one of Schmidt's cousins (I believe) a few weeks ago, or a comment on a post made by her cousin (I don't remember exactly), that said something along the lines of "I hope whoever they have in custody gets what they deserve".