Urban combat troops all over Boston Marathon. Who are these guys?

Closely look at the below images that were captured right after the Boston Bombings. You will notice ‘urban combat’ troops within Boston, helping out the police department. They are wearing typical desert army boots with a casual uniform to blend in. You will also notice a black suburban with some type of interceptor on the roof with government plates. How did they arrive so quickly? They must of knew something was going to happen right? Well this is where it gets twisted. A person who working in a building near the bombing said: “They told us there was a bomb located outside or near a building, we are evacuating people to safety. No more than a couple minutes later, the bomb went off a couple buildings over.” So that’s the rumor, that will only be found on a forum post and here. Now look at these images below, something seems weird: (Click to enlarge)