At which district did the multiple homicide case number JB120039 occurred?
multiple homicide refers to Count(case_number) > 1; district refers to district_name
SELECT T1.district_no, T1.district_name FROM District AS T1 INNER JOIN Crime AS T2 ON T1.district_no = T2.district_no WHERE T2.case_number = 'JB120039' GROUP BY T1.district_no, T1.district_name