Daily Kos has the story:

The same day that the NRA's GOP minions decided that it was okay for a violent criminal to shoot me in the face, I discover that the Boston investigators are blocked from doing a thorough investigation because of the NRA and their allies.

Identification taggants are used to make it possible to trace explosive materials after detonation back to the source of the bomb. It is simple, cheap, and allows tracing to a point of sale. Law enforcement, the ATF, and others have pleaded for it. Clinton tried to make it mandatory after the OK City bombing. Current experts say it would have helped with the current investigation.

Also MSNBC (start at 3:00):

No one, of course, is saying that gunpowder taggants would have prevented what happened in Boston; the thing that's driving everyone up the wall, and what connects the bombing to all of the rest of it, is the NRA's absurd insistence that shadowy gun ownership is somehow an American right.