WHAT WAS INSIDE THE PRESSURE COOKERS?

This is an 'uneducated' and uninformed theory as to the explosives contained within the Boston pressure cookers.

How about Claymore mines or C4?  The kill radius, if I remember correctly,  is 50 meters to the front in a 60 degree fan.  Projectiles from the Claymore are effective at 150 meters and can travel as far as 250 m.

 Claymores are curved.  One would fit nicely against the rearmost wall of a 5qt. pressure cooker, with the concave-face facing toward victims.  That said,  there is a direction that is faced toward the enemy, 'the target', and if the pressure cookers contained claymores as well as projectiles packed in front of them, ball bearings would be found as well as nails, tacks, brads etc... because claymores themselves are filled with steel balls.   These added projectiles would create more casualties.

This is just a thought.

Now if, by chance,  claymores were used in the pressure cookers, how did a civilian(s) get their hands on them? 

Claymores use a plastic explosive, and the explosion I saw on film looks like a plastic explosion to me.

The M18A1  uses the compound 'C4'.  It is a fragment device and each one contains 700 steel balls.

Just a thought.

Here's a video of the Claymore in use.  There are several explosions in this video, filmed at a distance.  Check out the last one.  It is the closest.  You can see the yellow-white blast-plume, just like that at the Boston Marathon.  Now that I've thought about it more, it probably wasn't a Claymore, but it sure appeared to be C4 that detonated. Compare this to the next video below of the Pressure cooker bomb in Boston.  

Do you see the similarities?