Transoceanic Cables

Some of the material Snowden downloaded in April 2012 while a Dell employee related to NSA collection from fiber-optic cables, including transoceanic cables, of large quantities of internet traffic and other communications, the sources said.

Snowden has said he left Dell for a job at Booz Allen Hamilton in Hawaii around March of this year, specifically to gain access to additional top-secret documents that could be leaked to the media.

Booz Allen Hamilton fired Snowden after he fled to Hong Kong with a trove of secret material. The company has said it is cooperating with a number of inquiries into Snowden's hiring and security lapses.

It is not clear whether Dell has taken similar steps.

"We are honoring our customer's request that we not comment on this matter," said Frink, the Dell spokesman.

Two US officials said the investigations into Snowden's activities confirmed that his downloading of sensitive information began at Dell. He is believed to have moved from Dell to Booz Allen with little time off in between.

In February 2010, while working for Dell, Snowden wrote in an internet technology forum, Ars Technica, that he was bothered by technology companies allegedly giving the US government access to private computer servers.

"It really concerns me how little this sort of corporate behavior bothers those outside of technology circles," Snowden wrote under the screen name "The True HooHA." "Society really seems to have developed an unquestioning obedience towards spooky types."

In addition to a Justice Department investigation, which has produced criminal charges against Snowden, US intelligence agencies are conducting an extensive inquiry to determine precisely what documents Snowden had access to, what he downloaded and how much damage his actions have caused.