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.
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?