More than 4.2 million people have security clearances for access to classified information, a number that vastly outstrips previous estimates and nearly rivals the population of metropolitan Washington.

(Michael Williamson for The Washington Post)

The official count is so much greater than previous estimates that it caught security experts off-guard. Steven Aftergood of the Federation of American Scientists said the new total is an “astonishingly large figure” and “another reminder of how quickly the national security bureaucracy has expanded over the past decade.”

Aftergood noted in his blog that just two years ago the Government Accountability Office estimated that about 2.4 million people held clearances. The Washington region’s overall population is just under 5.5 million.

The vast majority of clearance holders are federal employees or members of the U.S. armed services. But more than a million contractors also have access to classified information. Remarkably, nearly as many contractors hold top secret clearances (524,990) as do federal workers (666,008).

The report, which was mandated by last year’s intelligence authorization act, also examines how long it takes for various agencies to conduct background investigations and grant clearances. The National Security Agency appears to be particularly slow, with 192 cases that are more than a year old. At least one application has been under review for nearly three years.