Murder at Transit

A Threat has been made. A potential Serial Killer is targeting Transit. The City is put on Alert. Special Agents Thomas Banner and Joseph Sorelo from the TaskForce, have been assigned the case. Is it a Disgruntled Employee past or present? Is he/she an Imposter? Maybe, it's a Terrorist group, or some poor sole who wants to take out their frustration, after years of anonymity? This isn't a case of who done it, rather ... It's a case of ... Who Might Do It?