ALEXANDRIA, Va. — Prosecutors brought the first-ever federal terrorism charges against a law enforcement officer in the U.S., alleging Wednesday that a patrol officer with the D.C. region’s Metro Transit Police was caught in sting buying about $250 worth of gift cards for the Islamic State group. Nicholas Young, 36, of Fairfax, was arrested Wednesday at Metro’s headquarters in Washington and charged with a single count of attempting to provide material support to a terrorist group.