That's been my question. I can understand they didn't want to use their own cars so maybe they were out scouting for a vehicle. Maybe that was the reason for killing the MIT cop to take his car but then realized neighbors might ID them so got out of there. Still, it'd be cumbersome lugging around a pressure cooker, bombs and guns. You'd think they'd have had an exit plan but then most criminals aren't smart. They just thought they'd just blend back into their everyday lives la te da and no one would be the wiser. The younger one was even smiling in the marathon pics.
I don't think so. I think the cop was killed as a diversion, to draw attention away from their movements.