That's what I immediately thought - they had one with them and set it off during the fire fight with cops.
The FBI also searched the younger brother's dorm room at Dartmouth College and found a large incendiary device which they didn't identify in the article, plus they found BBs in that room.
A Malden taxi driver is being interviewed by Channel 7 today. He said he picked them up at the T station in Malden and drove them to Cambridge. He said he handled the back packs and they were heavy. That means they assembled them not far from where I live. That means they had help. And yes, there is a Mosque nearby.