AUDIE CORNISH, HOST:
Now a closer look at some evidence in the Boston bombing case. Initial reports from investigators suggest the Tsarnaev brothers acted alone, but it's still an open question whether they were the sole architects of the plot or whether they had help. Clues might be found in the bombs themselves. As we hear from NPR's Tom Gjelten, investigators want to learn whether the brothers could have built the bombs on their own or only with guidance from others.
TOM GJELTEN, BYLINE: From what is now known, it appears the brothers assembled a whole arsenal of explosives. Here's Watertown Police Chief Edward Deveau telling CNN last weekend what the suspects threw when they encountered the police Friday morning.
EDWARD DEVEAU: There's the three bombs that exploded, there's the two that weren't detonated, and in the car that he bailed out of I know there was at least one other explosive device in that car that they didn't use. So there was at least six bombs they had, if you will.
GJELTEN: Plus, the police found an explosive device in the Tsarnaev apartment in Cambridge. Evidently they had a whole bomb-making operation somewhere. So the question, could a couple of young men, whom their uncle considered losers, really have done this all on their own, without training or instruction or financial support?
MICHAEL BOUCHARD: Happens every day.
GJELTEN: Michael Bouchard is a former special agent at the Bureau of Alcohol, Tobacco, Firearms and Explosives, the ATF.
BOUCHARD: And it's amazing the number of bombs that law enforcement comes across every day in the United States. A lot of them are just young individuals, too much time on their hands obviously, who go on the Internet, experiment with these kind of things. And hopefully they just get caught and they don't end up getting injured.
GJELTEN: Bouchard, who retired as an assistant director at the ATF, says there are hundreds of websites that explain how to build a rudimentary bomb. First you need your explosive. You can make your own mixture or you can buy ready-made explosive off the shelf.
BOUCHARD: Things that are used in fireworks: black powder and smokeless powder are used in loading ammunition. You can go to gun stores. You can go to places like Wal-Mart, any of these kind of places that sell those things. It's sold for sporting purposes. You can then use it for bomb making if you choose to.
GJELTEN: The charging document released by federal authorities yesterday said the bombs used at the marathon had a hobby fuse. Normally, hobby fuses have to be lit with a match, though if there's a wire attached to the match, the bomber can light it with a little switch. The authorities have yet to explain how the bombs in Boston were actually lit.
Finally, there's the container. The Boston bombers used pressure cookers, but Bouchard says it could be anything that holds the explosive in a confined place.
BOUCHARD: If you ignite it, it will just burn, so what you need to do is put it in a container so that as it ignites, it burns, the hot gases build up, they can't go anywhere, thereby the device explodes.
GJELTEN: That's it, and this information is readily available. There seems to be widespread agreement in the law enforcement and intelligence worlds that the Boston bombers could easily have carried out that operation without help from outside. That is pretty frightening.
BRIAN FISHMAN: Terrorism has become an option in a way that I think it wasn't before.
GJELTEN: Brian Fishman is a counterterrorism research fellow at the New America Foundation.
FISHMAN: So when you look at an attack like this one, that succeeded in large part because of what seems to have been relatively rudimentary bomb instructions. What you worry is that other people that might be inclined to this kind of thing, but thought it was too hard to do may be willing to go out and experiment and think that they are empowered to do something.
GJELTEN: Leaders of terrorist movements like al-Qaida have urged their followers to do attacks on their own, in their own communities, against targets of their own choosing. What we don't know is whether the Boston bombing now fits that pattern. Tom Gjelten, NPR News, Washington.