It doesn't take a great leap of intellect to postulate that this is an infrastructure strike, not aimed at first responders nor at civilians. The announcements, and possibly gunshot noise would attract attention, and cause evacuation prior to the explosion, obviously NOT the work of people trying to rack up a body count.
Noted that the Airport shut down over related communications issues, and 911 services disrupted over a fairly large area, the nondescript building may have housed more critical communications infrastructure than noted in news reports. (which would not be surprising, because no one would want to discuss the intimate details of communications infrastructure with a public which would contain the bombers, who I'm pretty sure were targeting just that. There is no gain in announcing vulnerabilities to hostile forces, so while damage would be minimized in the press, any idea of severity would have to wait for reports of compromised services, if even that is reported outside emergency management circles.
