[SSN558]

"and saunter right back out through the dragnet without drawing any attention while carrying an assault rifle....."

The weapon is catched, "hidden" and then the shooter retreives it just prior to making the hit. After the shot the shooter runs a short distance and ditches the gun again. The gun is retrieved later or abandoned. If the gun gets found the shooter permanently disappears, they now have a case.

[AntiGuv]

I thought about that possibility as well, but it strikes me as distinctly against reason to think that there have been 10 assault weapons hidden around the DC area for the sniper/s to pick up, assemble, fire, and then hide again so well that the dozens of cops who swarm the scene haven't found a single one of those...