The U.S. Missile Defense Agency (MDA) has narrowed to four its list of candidates for a third interceptor site for its Ground-based Midcourse Defense system, the primary U.S. territorial shield, according to a Jan. 31 Pentagon press release

The sites are: Fort Drum, N.Y.; Naval Air Station Portsmouth SERE Training Area, Maine; Camp Ravenna Joint Training Center, Ohio; and Fort Custer Training Center, Mich. All are located in the eastern half of the United States.

The MDA has not made a decision on whether to build a third site, but will complete environmental impact studies of the four sites, as mandated by Congress in the 2013 National Defense Authorization Act. The studies will examine how a new missile interceptor site will impact things like land use, water resources, air quality, transportation and socioeconomics in and around the immediate area. The studies are expected to take 24 months, the release said.

Previously, the MDA also had been considering a site at Camp Ethan Allen Training Site, Vt. 

Currently the Ground-based Midcourse Defense system features two interceptor sites: Vandenberg Air Force Base, Calif., and Fort Greely, Alaska. U.S. Defense Secretary Chuck Hagel announced in March 2013 that the MDA would increase the number of interceptors at Fort Greely and begin looking for a third site in the United States, something Republican lawmakers have been pushing for in recent years.