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?