The nation’s fastest-growing cities may be in the Sun Belt, but the District and nearby Alexandria and Gaithersburg are gaining residents at a blistering pace, according to census data released Thursday.

D.C.’s population grew by 5.1 percent to 632,323 from 2010 to 2012, according to the Census Bureau. Gaithersburg was not far behind, growing by 4.9 percent to 62,794 residents over the same period. Alexandria’s population grew 4.5 percent to 146,294.

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