Several thoughts jump out:
1. Boston PD had enough LEO staff on site with existing personnel
2. Limited area, didn’t wish to add to the confusion at the time.
3. It was a crime scene and once injured were cleared had to be treated as such...NG aren’t trained to protect crime scenes....
I’m sure the offer was appreciated, just not necessary.
They were there for the potential WMD threat—Detect, respond and assist in WMD after-actions. Early detection means more informed medical response.