There's another thread here mentioning that an envelope with ricin was sent to a Republican senator. Different state, but the timing makes me think that they could be related. In which case it may be a radical left-winger, or someone independently insane.
The bombs were "pressure cooker bombs", homemade explosives packed into a pressure cooker and filled with bits of metal and ball bearings for shrapnel, hidden in two backpacks. Relatively low explosive power, relying on the shrapnel and fragmentation to kill people rather than causing property damage (no structural damage beyond blowing out adjacent building windows). One guy in the House of Representatives suggest black powder as the explosive, and the footage of the bombing does indeed look like a contained black powder explosion from the ones I've seen. Detonated by an egg timer.
It's a bomb which requires little skill to make as long as you can get the ingredients, suggesting an amateur. Al-Qaeda has yet to take credit, which is unlike them. Probably an individual radical, possibly a Boston local who would have known the significance of Patriots' Day. If it was Al-Qaeda related, I'd imagine it was an independent who fell for their propaganda and wanted to impress them.
Someone at the scene when the bombs went off was interviewed on the BBC news feed the other day and said it smelled distinctively of cordite, a low explosive that was used like gunpowder in the past.
I looked up the statement, but I can't find any info on that particular man's identity beyond "He called up and said so, yeah?"
Frankly, the explosion doesn't look like cordite. By no means should you take my opinion as expert; I only have some level of knowledge and experience when it comes to pyrotechnics, none of it put into practice (because I'm not that stupid) and very little of it "professional". But I do know some things. The initial blast in Boston was a dull reddish-orange, low-velocity propagation (I'll provide a picture below to explain), and large amounts of thick white smoke. It looks exactly like a contained black powder explosion.
This video opens up with a gunpowder explosion contained in a wooden barrel (less than a minute in, so no need to skip around). It looks extremely similar to the Boston Marathon bombs, as you can tell, albeit a much larger powder charge.
This is a far image of the explosion, and you can see the similarities.
As for the low blast wave, there's two pictures used for comparison that I'll place below. They're video screencaps highlighting the location of one of the bombs just before it goes off. The second picture has had the graphic content blurred out, but some people may not want to look anyway.
You'll notice how the areas around are intact, much like the reports that say that only the windows of adjacent buildings were shattered in terms of property damage. Nothing's been knocked over except for what was immediately next to the bomb, and there's surprisingly little debris around. It looks very much like a low explosive contained to turn it into a bomb (you need to contain black powder to get a true explosion, as it allows for pressure buildup that ruptures the container).
Also, black powder would be a perfect substance to use: it can be easily made at home by anyone with access to sulfur (can be purchased as garden supply, like
this link on Amazon), potassium nitrate (available in garden supply stores), and charcoal (burn some wood). It doesn't take a degree in chemistry to make; just follow an online recipe and use non-sparking tools and you'll get something that blows up. It's easy to ignite as long as you can make a spark, so it's reliable. Black powder bombs are ideal for wannabes who don't have the money, talent, or connections to make something more powerful like homemade plastic explosive (which is also harder to detonate).
