You ALL miss the most important point.They had all the citizens stay home or indoors, they had every cop in Boston and surrounding communities, Every Mass State cop,The Mass NG, Local and State police and NG forces from surrounding States, all the alphabet feds, ATF, FBI, probably even CIA when they were ID'ed as Chechen's.
They had APC's, and Hummers, cars, and vans, and aircraft .
All this force, and manpower, and technical assets spent about 20 hrs scouring a 20 block radius and they found,
SQUAT, diddly, nothing.
It was 1 observant citizen going for a butt after the "Forces of Light and Goodness" had given up, who noticed a blood trail heading toward his boat.