Complex sentence: Newtown police first entered the school at 9:45 am, approximately ten minutes after the first 911 call and approximately fourteen minutes after the shooting had started; four minutes after the last shot is heard when the shooter is believed to have committed suicide.
Split into simpler sentences: Newtown police first entered the school at 9:45 am, approximately ten minutes after the first 911 call and approximately fourteen minutes after the shooting had started. Police entry into the school is approximately four minutes after the last shot is heard; when the shooter is believed to have committed suicide.