Complex sentence: The detectives mentioned a practical joke by medicine students as one of the possibilities, the academic year had just started and the students have relatively easy access to dead bodies, but fear that they are dealing with a serial killer.
Split into simpler sentences: The detectives mentioned a practical joke by medicine students as one of the possibilities, the academic year had just started and the students have relatively easy access to dead bodies. The police fear that they are dealing with a serial killer.