Former National Security Agency contractor Edward Snowden impersonated the electronic identities of top NSA officials in order to access the highly classified documents he leaked to the press, NBC News reports.

While the NSA says it doesn’t know exactly what Snowden took, reports the publication, it estimates that he stole as many as 20,000 documents from the agency.

By impersonating senior officials in the agency, Snowden was able to access documents not even available to him with his “top secret” clearance.

Snowden’s job as a systems administrator allowed him remote access to the NSA’s intranet, as well as a level of access not available to regular intelligence analysts.

He could also “create and modify user profiles for employees and contractors,” reports NBC News.

“He also had the ability to access NSAnet using those user profiles, meaning he could impersonate other users in order to access files. He borrowed the identities of users with higher level security clearances to grab sensitive documents,” reports the publication.

Among the documents that Snowden stole from the U.S. government was the 2013 fiscal year U.S. intelligence “black budget,” which contained details about how the U.S. government deployed a myriad of capabilities across the intelligence community to hunt and kill Osama bin Laden.