British and U.S. intelligence officials say they are worried about a doomsday" cache of highly classified, heavily encrypted material they believe former National Security Agency contractor Edward Snowden has stored on a data cloud. The cache contains documents generated by the NSA and other agencies and includes names of U.S. and allied intelligence personnel, seven current and former U.S. officials and other sources briefed on the matter said. One source described the cache of still unpublished material as Snowden's "insurance policy" against arrest or physical harm. U.S. officials and other sources said only a small proportion of the classified material Snowden downloaded during stints as a contract systems administrator for NSA has been made public. Some Obama Administration officials have said privately that Snowden downloaded enough material to fuel two more years of news stories. "The worst is yet to come," said one former U.S. official who follows the investigation closely. Snowden ... is believed to have downloaded between 50,000 and 200,000 classified NSA and British government documents. [It is] estimated that the total number of Snowden documents made public so far is over 500. Glenn Greenwald, who met with Snowden in Hong Kong and was among the first to report on the leaked documents for the Guardian newspaper, said the former NSA contractor had "taken extreme precautions to make sure many different people around the world have these archives to insure the stories will inevitably be published."