John Stark is a decorated military solider who later worked as a Green Beret for the CIA. He later became "the Surgeon's apprentice." While in a combat medic training program at For Stewart, Stark meets Charles Hoyt who, then, was using an assumed name. Some time later, he is sent to Afghanistan for three tours as a part of a direct action force. He was also assigned to medical care specialists' Unit 618, the same unit as Charles Hoyt, which is how Jane Rizzoli figure out their connection.
In Kabul, Afghanistan, Stark begins his killings using the same m.o. (modus operandi) as Hoyt. During the FBI's initial investigations, they assumed that it was war killings, given that the area was a war zone at that time.
After his Afghanistan tour, he went back to Boston and freed his mentor, and they both went for Rizzoli. She fights back and uses the stun gun against them. She ends up shooting Stark twice in the chest as self-defense, therefore killing him, 32 at that time.