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More than a fifth of Federal Air Marshal Service's field offices will be closed over the next two years, government sources told NBC News late Tuesday.

Pittsburgh, Tampa, Cleveland, Phoenix, San Diego and Cincinnati will lose their bases, leaving 20 sites across the country. Air marshals will continue to fly in and out of the cities which are losing field offices.

The cities were chosen because of changing airline priorities and mergers with flight patterns and passengers loads also being taken into consideration.

"We need to be where the threat is," one FAMS manager told NBC News.

The highly trained operatives blend in with passengers on aircraft and are tasked with spotting and neutralizing potential terror threats.

The government has not divulged the number of U.S. air marshals but several thousand are believed to be employed.