WASHINGTON — Edward J. Snowden’s employer, Booz Allen Hamilton, has become one of the largest and most profitable corporations in the United States almost exclusively by serving a single client: the government of the United States.

Over the last decade, much of the company’s growth has come from selling expertise, technology and manpower to the National Security Agency and other federal intelligence agencies. Booz Allen earned $1.3 billion, 23 percent of the company’s total revenue, from intelligence work during its most recent fiscal year.

The government has sharply increased spending on high-tech intelligence gathering since 2001, and both the Bush and Obama administrations have chosen to rely on private contractors like Booz Allen for much of the resulting work.

Thousands of people formerly employed by the government, and still approved to deal with classified information, now do essentially the same work for private companies. Mr. Snowden, who revealed on Sunday that he provided the recent leak of national security documents, is among them.