He took a course that trains security professionals to think like hackers and understand their techniques, all with the intent of turning out “certified ethical hackers” who can better defend their employers’ networks.

But the certification, listed on a résumé that Mr. Snowden later prepared, would also have given him some of the skills he needed to rummage undetected through N.S.A. computer systems and gather the highly classified surveillance documents that he leaked last month, security experts say.

Mr. Snowden’s résumé, which has not been made public and was described by people who have seen it, provides a new picture of how his skills and responsibilities expanded while he worked as an intelligence contractor. Although federal officials offered only a vague description of him as a “systems administrator,” the résumé suggests that he had transformed himself into the kind of cybersecurity expert the N.S.A. is desperate to recruit, making his decision to release the documents even more embarrassing to the agency.

“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.”