The unprecedented leak of top-secret documents by National Security Agency contractor Edward Snowden raises far-reaching questions about the government’s rush to outsource intelligence work to contractors since the Sept. 11, 2001, terrorist attacks.

Never before have so many U.S. intelligence workers been hired so quickly, or been given access to secret government information through networked computers.

In recent years, about one in four intelligence workers has been a contractor, and 70 percent or more of the intelligence community’s secret budget has gone to private firms.

Booz Allen Hamilton, which hired the 29-year-old Snowden three months ago to work at the NSA, has been a leader among more than 1,900 firms that have supplied tens of thousands of intelligence analysts in recent years, including technologists and field spies.

But in the rush to fill jobs, the government has relied on faulty procedures to vet intelligence workers, documents and interviews show. At the same time, intelligence agencies have failed to hire enough in-house government workers to manage and oversee the contractors, contracting specialists said.

On Monday, lawmakers said they would be examining Snowden’s hiring and the growing use of private companies for intelligence work.

“We’ll be going over every inch of this,” said Rep. Adam B. Schiff, D-Calif., a member of the House Intelligence Committee who expects confidential briefings on the leak in the next few days. Public hearings are likely as well, he said.

Schiff said the committee long has worried about the cost of outsourcing but now would be examining the security risks more closely. “Now I think we’ll be looking that through an entirely different lens,” he said.

Intelligence officials, government auditors and contracting specialists have warned for years that the vulnerability to spies and breaches was rising, along with contracting fraud and abuses.

“When you increase the volume of contractors exponentially but you don’t invest in the personnel necessary to manage and oversee that workforce, your exposure increases,” said Steven Schooner, co-director of the government procurement law program at George Washington University. “This is what happens when you have staggering numbers of people with access to this kind of information.”

The reliance on contractors reflects a massive shift toward outsourcing over the past 15 years, in part because of cutbacks in the government agencies and commitment to smaller government by the George W. Bush administration.

Most of the work went to the largest contractors, including Booz Allen Hamilton, which had $5.8 billion in revenue last year. Almost all of Booz Allen’s work was for the government, and nearly a quarter of that was for intelligence agencies.

In the first few years after 2001, when the competition for qualified job candidates was the fiercest, it was not unusual for companies to give signing bonuses of $30,000 or a new car for workers with top-secret security clearances.

By 2010, the overall intelligence budget had grown by 250 percent since 2000. Nowhere was the growth larger than at the NSA. The budget there doubled, as did the physical infrastructure. The hidden Fort Meade, Md., complex includes as much square footage as the Pentagon and is surrounded by 112 acres of parking lots, according to military construction documents filed with Howard County, Md. Ten thousand employees are to be added in the next 15 years, according to the plans.

Many of the NSA’s contractors are located in the 285-acre National Business Park, which is connected to the agency by a private road. Booz Allen shares the skyline there with other giants: L-3 Communications, Northrop Grumman and SAIC, to name a few.

By the mid-2000s, all of the intelligence agencies had become dependent on private contractors such as Snowden — who says he made $200,000 a year — to perform everything from information technology installation and maintenance to intelligence analysis and agent protection.