“If he’s looking inside U.S. government networks for foreign intrusions, he might have very broad access,” said James A. Lewis, a computer security expert at the Center for Strategic and International Studies. “The hacker got into the storeroom.”

In an age when terabytes of data can be stashed inside palm-size devices, the new details about Mr. Snowden’s training and assignments underscore the challenges that the N.S.A. faces in recruiting a new generation of free-spirited computer experts with diverse political views.

Mr. Snowden, who is now marooned at an airport in Moscow waiting to see if another country will grant him asylum, has said he leaked the documents to alert the public to the sweeping nature of the American government’s surveillance. He took a job as an “infrastructure analyst” with Booz Allen Hamilton in April at an N.S.A. facility in Hawaii, he has said, to gain access to lists of computers that the agency had hacked around the world.

Mr. Snowden prepared the résumé shortly before applying for that job, while he was working in Hawaii for the N.S.A. with Dell, the computer maker, which has intelligence contracts. Little has been reported about his four years with Dell, but his résumé, as described, says that he rose from supervising computer system upgrades for the spy agency in Tokyo to working as a “cyberstrategist” and an “expert in cyber counterintelligence” at several locations in the United States.

In what may have been his last job for Dell in Hawaii, he was responsible for the security of “Windows infrastructure” in the Pacific, he wrote, according to people who have seen his résumé. He had enough access there to start making contacts with journalists in January and February about disclosing delicate information. His work for Dell may also have enabled him to see that he would have even more access at Booz Allen.