Jonas Walker was a criminal with a long rap sheet. He was under FBI observation as a potential active shooter, with Special Agent Grant MacLaren taking a particular interest in his activities.
Walker hasn't left his house in a week, which prompted MacLaren to think surveilling him was a waste of time. MacLaren was proven wrong, however, when he found Walker's body in the 12th floor of an abandoned building, along with four people he'd been following who were communicating on an encrypted chatroom.
The four identified themselves as Travelers from the future, and they preemptively eliminated Jonas Walker. Walker historically went on a shooting rampage, while MacLaren died when he fell down an elevator shaft while pursuing Walker. MacLaren's consciousness was subsequently overwritten by Traveler 3468 around his historical time of death, and the Traveler took credit for neutralizing the criminal when backup police forces arrived. (S1E01: Travelers, S1E02: Protocol 6)