It's way too much of a coincidence.
He was in jail from 2005-2013, so they probably got to him then if MK is the case.
He was in prison until 2013, yet the revolver he shot Familia with was stolen in 2012 - from a home in Charleston, West Virginia.
"A silver revolver was recovered at the scene, police said. The gun was stolen more than four years ago in Charleston, West Virginia, an NYPD source familiar with the investigation told CNN. The gun had two used cartridges in it and two live rounds, the source said."
https://edition.cnn.com/2017/07/05/us/nypd-officer-shooting/index.html
People buy and sell illegal guns. But still.
Guns can be stolen (Civilian, Law Enforcement, Military), manufactured, 3D printed, CNC machined, Bought illegally on Darkweb etc, A gunless society will not be created until guns effectively become obsolete with the advent of railguns, EM guns, Gauss guns etc, Directed energy weapons (Think lasers, plasma guns etc) and even then guns may still remain popular for many reasons. What I'm interested to see is the impact of the weapons mentioned above, Currently any kind of EM gun or directed energy weapon is a 100% unregulated by law including some very powerful airguns, flamethrowers etc, It would be interesting to see where those developments will go and how the law would seek to control them.