HOUSTON -

We are learning more about at least one of the devices used in the Boston bombings. A lid from a pressure cooker was discovered on top of a nearby roof, showing the power of the blast.

The pressure cookers are widely available and retired CIA officer David Adler says they've been used as bombs for decades.

“The idea is to try and contain or try to hold it in place just a small amount of time which will cause the explosion to push out the shrapnel and the other materials inside at a faster velocity that may cause more damage to people standing nearby,” said Adler.

Investigators believe the pressure cooker was concealed in a black bag. Black powder, nails, and ball bearings were inside the cooker. 

A circuit board and a battery pack were also recovered at the scene, which are believed to have been used as a triggering mechanism.

Because all of the items are widely available, Adler said determining who made the bomb will be difficult.

“Unfortunately, it's one of a variety of explosives that can be made very cheaply, so the range of people who might use this is as varied as the range of people who have a grievance against the U.S., or Americans, or any particular organization,” said Adler.