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Some are suggesting that an IED (improvised explosive device) was used for the bombing.  

Improvised explosive devices are homemade bombs made with every day materials that one can find at the local hardware store.  Typically, large purchases of certain chemicals that can be used to make bombs set off an alert, but can be found easily if purchased carefully.

News officials from WBZ news in Boston are stating that they appear to be homemade devices.

Federal official when asked if these were bombs: "Hell yes, what else would they be?" 

One soldier at the scene who heard the blast and witnessed it himself said that he had seen IED attacks in Afghanistan, and that this was exactly reminiscent of what happened to him there.