Homeless
An Iraq War vet who has been living rough in the park across the street for the past year or so. Not too broken (if he was too mentally damaged, he would probably not be an interesting enough character), but has fairly severe PTSD, which slowly ruined his life.
He has gotten to know the folks in the front office (receptionist? cleaning staff? etc), and actually struck up a bit of a friendship with several people in the office, including the CEO, who was sufficiently self-consciously patriotic to want to support war vets. He had gotten into the habit of wandering in during quieter hours and bumming some coffee and a donut from the lunch room.
The CEO had recently been talking with him about possibly even getting a real job there, helping with the prototyping -- his background was machinery and maintenance, so he knows his way around a machine shop. He has actually been kibitzing a little on Project Sureshot, helping the engineers understand what it is like in a real urban-combat environment. That was hard, thinking about it, but talking it through in such a clinical way was actually a bit calming.
But he was in the lunchroom when Seamus went on his rampage; he started screaming when he heard the gunshots and jumped under the table. Seamus heard him came in and tried to calm him down, reassuring him that this wasn't about him, but he got physically violent (possibly punched Seamus?), so Seamus gave up and went back to what he was doing.
At this point, the police are confused and suspicious about what this homeless man was doing in the building. At least one of them should be set up to be thinking that he was obviously some sort of accomplice.
