Hypothesis: A "bomb searching drill" was staged during the marathon.
A part of the "Civil Support Team" of the National Guard (CST) plays the role of the "badies": about ten guys with backpacks containing some sort of harmless explosives to make some smoke etc., just enough to be realistic. Their task is to walk around and find a suitable place and moment to put down their backpack, i.e. without attracting attention and not easy to find for the dogs.
The task of the Boston police is, of course, to find the bombs before they go off.
Now either two of the CST players were traitors - or someone replaced the loads of two backpacks without telling the CST guys.
Remind the meanwhile famous pictures with the two CST guys standing in the middle of the road after the bombings. They are obviously not trying to hide who they belong to. They also are certainly aware that they are pursued and still observed by hundreds of camera eyes. In other words: they certainly are not behind the terror attack. They look pretty perplexed. They were playing a game, but now the game has turned into a catastrophe. That's also the reason why they're not helping: they're waiting for orders how to proceed with the game.
And then the CST SUV arrives, the "badies" abort the game, gather at the car and move away.
But who replaced the loads of the two deadly backpacks? This is another question, and it cannot be answered yet.
