First, Sandy Hook/Newtown wasn't terrorism. By definition it was not 'the use of violence and threats to intimidate or coerce, especially for political purposes.'[1]. It was more of a random spree killing. As for Boston, that's a terrible example because it was committed by Chechen Muslims.
Second, we don't know if there was a false positive. Maybe some cleaner or the bed bug spray do have legitimate bomb-making uses. There's a lot of conjecture in the story and even more in this comment thread.
> Maybe some cleaner or the bed bug spray do have legitimate bomb-making uses.
With bed bug spray you can set up a decent fire, maybe even blast an apartment, cf. my other comment. This is why it's not easy to buy it and that's why you should read the manual. Lovely to hear how many people use highly flammable chemicals without reading the manual.
Second, we don't know if there was a false positive. Maybe some cleaner or the bed bug spray do have legitimate bomb-making uses. There's a lot of conjecture in the story and even more in this comment thread.
[1] http://dictionary.reference.com/browse/terrorism?s=t