Sanjana's Roomie Murder Mystery

Riddle

As Sanjana entered her house and went to hang her coat, when her roommate yelled, "No, John! Do not attempt it!" There was a shot, and she heard her roommate fall. When she stepped into the kitchen, she saw the girl and the rifle on the ground. A cop, a doctor, and a lawyer were standing nearby. Who murdered Sanjana's roomie, and how did she find out know?

    She identified the cop since John was mentioned on his badge.