The LaSalle Bank building is on fire

Here are close up pictures of the LaSalle Bank building, from the Post Office on Dearborn, taken at about 9PM central:
In the early evening, I looked out my window and saw a helicopter hovering in the area of LaSalle and Adams.

I went outside and started walking towards the Target on Roosevelt. As I walked, I realized that more helicopters were starting to show up, but not near the LaSalle Building. There were at least 5 helicopters clearly and systematically searching buildings and terrain south of Adams, both east and west. (The image quality isn't good, just look for a bright spot in the sky where there shouldn't be one.)

I can't figure out why all these helicopters showed up and searched buildings that were in no danger whatsoever from the LaSalle Bank fire. Is this standard procedure? Or did the authorities have some information here that wasn't made publicly available? My first thought was "they might have a tip about a terrorist threat". Anyway, I made a map with the observed helicopter locations (blue dots) relative to the LaSalle Bank building (red star).