One could argue that mass shootings occur every day in Chicago, the most Firearm Free city in the nation.
I think it is a tossup between Washington, D.C. and New York City as the most firearm free city in the nation. Both have such onerous requirements to obtain a license, firearms are practically banned.
At least in theory, Chicago is like Detroit, where it is shall issue for a concealed weapons permit, and in Illinois it is shall issue for a Firearm Owners Identification (FOID)Card.
It is a safe bet, however, that the vast majority, if not practically all, shootings in Chicago are by prohibited persons, making the restrictions on law abiding citizens all the more moronic.