Discuss with the interviewer what your long term career goals look like, and show your dedication to get there. Be sure to display to the interviewer that you plan to stay in this new role for the longer term,
"As a Police Officer, my goal has always been to become a Detective in a Homicide Unit. In 5 years I would definitely like to see myself there and leading a team myself. I am absolutely willing to complete any additional training and coursework required to get me there."