Top 120 Cities at Risk for NBC Attack is a single page PDF that shows (obviously) the author’s idea of the top cities at risk for an attack.
I don’t have any information on the author or how they made the determination of what cities made the list.
That being said, as I look at the list, the cities mentioned are probably ones I would attack if I was an enemy of the US.
This list is not necessarily tied to military bases. Target factors could include population density, cultural centers, proximity to nuclear plants or NBC waste sites,
Also is a city is important as a transport hub – a shipping / trucking route/port. If it has important or symbolic industry (logging for eco terrorists as an example), dams, communications hubs and other critical infrastructure sites, plus a variety of other considerations not listed.
This list is to increase awareness. However, you need to understand that just because you don’t live in or near a city on this list you are not immune to terrorism remember the previous discussion of the cult influenced bioterrorism attack to influence a local election? That happened in a small Oregon town.