Booz Allen’s cyber facility in Maryland is one of nine virtually-connected centers and labs in the Cyber Solutions Network, each equipped with the tools and expertise used to counter cyber threats. (Jeffrey MacMillan/For The Washington Post)

The unprecedented leak of National Security Agency secrets by an intelligence contractor, including bombshells about top-secret programs to collect telephone records, e-mail and other personal data, was probably an inevitable consequence of the massive growth of the U.S. security-industrial complex.

Edward Snowden, the 29-year-old man who identified himself as the source behind stories in The Washington Post and the Guardian newspapers, has worked at Booz Allen Hamilton and other intelligence contractors. Before entering the private sector, he says he held a series of technical jobs at the Central Intelligence Agency.

In a statement Sunday, Booz Allen said, “Booz Allen can confirm that Edward Snowden, 29, has been an employee of our firm for less than 3 months, assigned to a team in Hawaii. News reports that this individual has claimed to have leaked classified information are shocking, and if accurate, this action represents a grave violation of the code of conduct and core values of our firm. We will work closely with our clients and authorities in their investigation of this matter.”

Snowden was among tens of thousands of private intelligence contractors hired in the unprecedented push to “connect the dots” after the Sept. 11, 2001, terrorist attacks. They work side by side with civil servants as analysts, technical support specialists and mission managers. An unknown number have access to secret and top-secret material.

Several years ago, the Office of the Director of National Intelligence estimated that almost one in four intelligence workers were employed by contractors.

The growing reliance on contractors reflects a massive shift toward outsourcing over the past 15 years, in part because of cutbacks in the government agencies. It has dramatically increased the risk of waste and contracting abuses, government auditors have found, in part because the government has repeatedly acknowledged that it does not have a sufficient workforce to oversee the contractors.

But given the threat of terrorism and the national security mandates from Congress, the intelligence community had little choice. In a briefing presentation several years ago, the ODNI estimated that 70 percent of the intelligence community’s secret budget goes to contractors such as Booz Allen Hamilton.

“We Can’t Spy . . . If We Can’t Buy!” the briefing said.

The former director of naval intelligence, retired Rear Adm. Thomas A. Brooks, said in a report in 2007 that private contractors had become a crucial part of the nation’s intelligence infrastructure.

“The extensive use of contractor personnel to augment military intelligence operations is now an established fact of life. . . . It is apparent that contractors are a permanent part of the intelligence landscape,” he said.

Since Sept. 11, more than 30 secure complexes have been constructed to accommodate top-secret intelligence work in the Washington area. They occupy the equivalent of almost three Pentagons, about 17 million square feet.

An examination by The Post in 2010 found that 1,931 private companies work on programs related to counterterrorism, homeland security and intelligence in about 10,000 locations across the country.

At the same time, tens of billions of dollars have been spent by intelligence agencies and the Department of Homeland Security on computers, networks, satellite systems and other technology to collect and mine information. Those systems have been shown repeatedly to be vulnerable — to attacks and exploitation by both hackers and insiders.

The most notable example is Army Pfc. Bradley Manning, 25, who downloaded more than 700,000 classified documents from secure military networks. He is accused of passing them on to the anti-secrecy Web site WikiLeaks, in what was the largest leak of classified documents in U.S. history.

Manning is being court-martialed on 22 charges, including aiding the enemy, and could face life in prison. He obtained access to the documents while working as a low-level intelligence analyst on assignment in Baghdad, during the war in Iraq.

The documents leaked by Snowden have been fewer but far more sensitive, including top-secret material. None of Manning’s documents were top secret.

Snowden’s release of scores of pages of top-secret material about a data surveillance program, code-named PRISM, and an NSA program to collect information about millions of phone calls from Verizon underscores one of gnawing worries in the national security community: The more people allowed in the top-secret tent, the higher the risk of leaks.