The FBI has released images of the remains of a "pressure cooker" bomb used in the Boston marathon attack and the backpack it was hidden inside, ABC reports. The second bomb was also hidden in a backpack, but it's not clear whether it was also made from a pressure cooker, the FBI says. Investigators say the bombs—packed with BBs, nails and pellets to maximize shrapnel injuries—may have cost less than $100 to build and were made from ingredients so ordinary that tracing them may be impossible, reports the Washington Post.
Investigators believe the bombs were triggered with "kitchen-type” egg timers instead of cell phones and used a common explosive like black gunpowder, the New York Times reports. "The black powder, or smokeless powder, you can buy in Walmart or gun shops. It’s used for hobbies,” a former ATF assistant director says. “All you need is a fuel and an oxidizer, and the Internet is great for explaining how to do this." Experts say that while the simple bomb design suggests the attack was the work of an amateur, it could also be the work of a master bombmaker expertly covering his tracks. (Read more Boston Marathon bombing stories.)