Washington: United States intelligence officials say they have determined that Edward Snowden gained access to a huge trove of the country's most highly classified documents using inexpensive and widely available software to "scrape" the National Security Agency's networks, and he kept at it even after he was briefly challenged by agency officials.

Using "web crawler" software designed to search, index and back up a website, Mr Snowden "scraped data out of our systems" while he went about his day job, according to a senior intelligence official. "We do not believe this was an individual sitting at a machine and downloading this much material in sequence," the official said. The process, he said, was "quite automated".

Edward Snowden: Gained access to highly-classified documents using a web crawler. Credit:Getty Images

The findings are striking because the NSA's mission includes protecting the nation's most sensitive military and intelligence computer systems from cyber attacks, especially the sophisticated attacks that emanate from Russia and China. Mr Snowden's "insider attack", by contrast, was hardly sophisticated and should have been easily detected, investigators found.

Moreover, Mr Snowden succeeded nearly three years after the WikiLeaks disclosures, in which military and State Department files, of far less sensitivity, were taken using similar techniques.