We have had the Sandy Hook shooting (December 2012) and the Boston Bombing (April 2013) recently.
So, what did they have in common?
All of the attackers did not legally posess some type of firearm (shooter 2, bombers) which they used.
Furthermore, the bombers used were pressure cookers with ball bearings and low tech explosives (gunpowder, ammonium nitrate fertilizer and diesel, etc.).
The problem with banning guns to prevent people like the Boston Bombers from getting weapons is that it does not work. The Boston Bombers used (relatively) common materials to make their bombs, most of which would be difficult to regulate (you want a pressure cooker? Got to pass a background check...), and I don't see that happening.
A pressure cooker does not kill human beings by itself. It needs a mind behind it, a mind like that of the Boston Bombers. One of them had even been interviewed by the FBI.
So, why do we have such an inept agency? Why are taxpayers paying the FBI? Really, why?
The FBI knew about one of the bombers (Russia had sent some information linking the older bomber to some "known" sympathizers of terrorism.
On the other hand, the FBI is supposed to work on other crimes (crimes which cross state borders).
Here is a problem for the FBI however; what do they do when the object of their investigation is anonymous because they used Tor/botnets to anonymize themselves? They know that someone has been looking at how to make (more) powerful bombs, but not who it is, for example.
Here is another problem for the FBI; most people can figure out how to make bombs on their own, if they are willing to a) read military fiction/online news and how-to's or b) experiment.