At the beginning of the episode in Boston.....with the curious note of some Saudi college student being detained....I had a belief that this might all lead to him and was just one Muslim guy. In the last couple of hours, you can sense that the FBI isn't getting any connection out of this guy, and nothing in house leads onto more clues. His involvement? Probably just an innocent bystander.
So who did it?
Possibly North Korean agents? Well....it wouldn't be difficult for a couple of guys to pick up the materials....manufacture the bombs in the back of a rental van, and place them at the right spots.
Possibly some nutcase 16-year old kid? Everything used could have been bought for less than a thousand bucks. There's probably 100k teenage kids who would have been smart enough to do it in the US.
Possibly some Latino drug gang? There's not much to gain except the feds on your tail. If there's no profit, why bother?
Possibly the Aryan Brotherhood guys....just demonstrating their abilities? It wouldn't surprise me, but they would have left some calling card to show their involvement.
In the end, I think you are looking for one guy. He's bought three pressure cookers from some Wal-Mart or Dollar-General Store. He's probably a guy with an agenda but always talking up the fairness of life. He's smart.....maybe a year or two of college. He might even have a tour in the Army within his background. The key thing? He's made a mistake or two. Some photo is going to pin him with a back-pack and by this weekend....the nation will be searching for him.