Tamerlan’s 2009 assault charge for striking his then-girlfriend meant he could not receive a firearm of any kind legally under federal laws. Dzhokhar’s age, 19, meant that he could not qualify for handguns under federal law. Neither man could qualify for a Massachusetts pistol permit for these same reasons, and the state’s ban on assault weapons — signed by Governor Mitt Romney — meant such rifles were not available for sale in the state without being heavily modified to comply with state restrictions.

Indeed, if the rifle is indeed correctly identified as an M4, it was almost certainly stolen from the military or police, as none have been manufactured for the civilian market since the Hughes Amendment became law in 1986.

Despite the state and federal laws, the suspects obtained firearms, and when they couldn’t acquire as many as they thought they might need, they apparently attempted to acquire more firearms by killing an agent of state they knew would be armed in order to rob him of his weapon.

This a tactic of the guerrilla fighter as old as war itself — the tactic was even encouraged by our own government for resistance groups in multiple wars. The FP-45 “Liberator” from World War Two and the CIA’s Vietnam-era “Deer gun” were both crude single-shot assassination pistols designed for the explicit purpose of murdering police and soldiers in order to obtain their duty weapons.

And therein lies the problem: to completely remove arms from the reach of criminals and terrorists, we have to disarm the police as well as law-abiding citizens. How well do you think that will sell down at the station?