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.

Those three were the only representatives from the mid-Atlantic to crack the top 100 fastest-growing cities with populations of 50,000 or more; no cities in the Northeast made the cut. Most were in the South and Mountain West, with Texas alone accounting for 27 cities on the list.

City limits
Fastest-growing U.S. cities, population greater than 50,000
Population, 2010 Population, 2012 Population change Percent change
1. Cedar Park, Texas 51,739 57,957 6,218 12.0
2. San Marcos, Texas 44,894 50,001 5,107 11.4
3. South Jordan, Utah 50,418 55,934 5,516 10.9
4. Georgetown, Texas 47,416 52,303 4,887 10.3
5. Frisco, Texas 117,003 128,176 11,173 9.5
6. Pasco, Wash. 59,781 65,398 5,617 9.4
7. McKinney, Texas 131,103 143,223 12,120 9.2
8. Irvine, Calif. 211,906 229,985 18,079 8.5
9. Lehi, Utah 47,460 51,173 3,713 7.8
10. Alpharetta, Ga. 57,500 61,981 4,481 7.8
59. The District 601,723 632,323 30,600 5.1
64. Gaithersburg 59,880 62,794 2,914 4.9
77. Alexandria 139,966 146,294 6,328 4.5
Source: U.S. Census Bureau

"Clearly, Texas is near the top in terms of growth in the United States," said William Frey, a senior fellow at the Brookings Institution's Metropolitan Policy Program. "It's one of the states that gets a good number of immigrants as well as migrants from the rest of the country."

Texas was insulated from much of the foreclosure crisis, Frey added, mainly because housing prices there never reached the heights they did in other states in the lead-up to the bust. D.C., with Alexandria and other inner-ring suburbs, weathered the recession thanks largely to a reliance on government and government-related jobs, which has helped keep growth rates up.

"D.C. and Alexandria are part of the inner-core growth," Frey said. "They both benefited short term from the recession and housing crunch and will continue to benefit."

While in the Washington area, Gaithersburg remains an outlier in that regard. Mayor Sidney Katz attributed the growth to the city's businesses and school system.

"We're here every day, so you don't notice it growing on a daily basis, but over the years we have seen Gaithersburg grow," he said. "Good businesses bring good jobs. People come to work here and know it's a nice area to live in."

Another reason is increased immigration. Montgomery County brought in 8,659 immigrants between 2011 and 2012, enough to push the county past the 1 million resident mark for the first time.

"We have seen an increase in population, mainly Hispanic, though we also have seen many immigrants from Africa," said Gaithersburg resident Angelica Thomson, who works for the city's Catholic Charities office, which provides services to new immigrants. "The cities are very expensive to live in, so people try to look for suburban places."