I think intel led to knowledge a meeting was taking place in that room or something important was stored there.
Facts:
One guest arriving after the window/residue is present. Referred to as "last" guest implying others already present.
Window opened in advance with expectation of pressure blast.
Supposed proof of arrests we have asked for.
Theory:
We knew some bad guys were meeting there. The room was prepared for a raid in advance of the meeting (window opened, etc).
Meeting started, room raided, people arrested. Residue outside window now from flash bangs or other activity in the room.
Last guest arrested once they show up, late to meeting or expected to be late.
Operation successful at gathering up mid level players in crime syndicate. Intelligence gathering operation. Either the people or what they had on them.
No point in discussion though I imagine but quite the cliff hanger.