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, its a Terrorist group, or some poor sole who wants to take out their frustration, after years of anonymity?

This isnt a case of who done it, rather

Its a case of Who Might Do It?