WASHINGTON -- Despite U.S. intelligence officials’ repeated denials that the National Security Agency is collecting the content of domestic emails and phone calls, evidence is mounting that the agency’s vast surveillance network can and may already be preserving billions of those communications in powerful digital databases.

A McClatchy review of public records, statements by Obama administration officials and interviews with cyber and telecom security experts lends credence to assertions that the capability for such surveillance exists.

– FBI Director Robert Mueller told a Senate committee on March 30, 2011, that “technological improvements” now enable the bureau “to pull together past emails and future ones as they come in so that it does not require an individualized search.”

– The administration is building a facility in a valley south of Salt Lake City that will have the capacity to store massive amounts of records – a facility that former agency whistleblowers say has no logical purpose if it’s not going to be a vault holding years of phone and Internet data.

– Security experts, including a former AT&T engineer, say that the NSA has tapped into fiber-optic cables carrying phone and Internet data in cities across the country.

Glenn Greenwald, a columnist for the British newspaper The Guardian who reported on the disclosures of NSA leaker Edward Snowden, said in a speech over the weekend that an upcoming story will describe a classified document that “talks about how a brand new technology enables the National Security Agency to redirect into its own repositories 1 billion cellphone calls every single day.”

“What we are really talking about here is a globalized system that prevents any form of electronic communication from taking place without it being stored and monitored by the National Security Agency,” Greenwald said in a webcast to the Socialism Conference in Chicago. “It means they’re storing every call and have the capability to listen to them at any time.”

Greenwald’s assertions and hints on the public record still offer only a cloudy picture, and the NSA has not disclosed that it is building a storehouse for trillions of digital recordings of phone calls and emails.

The NSA collected bulk data on Americans’ emails for years before that program was shut down in 2011 when intelligence officials could not provide evidence of its effectiveness, Democratic Sens. Ron Wyden of Oregon and Mark Udall of Colorado said in a statement Tuesday. The agency now collects data showing who sent and received emails under a narrower program focused on foreign terrorism suspects.

But the continuing revelations have poured kerosene onto a fiery, long-running post-9/11 debate over whether the NSA, in a scramble for tiny clues that might prevent terrorism, is breaching citizens’ Fourth Amendment rights to privacy.

Jonathan Turley, a George Washington University law professor, charged that what’s already known shows that the NSA has violated the Constitution and created “the greatest threat to privacy in the history of this country. . . . They have created a system that for the first time makes every citizen visible to the government at the simple pressing of a button.”

Michael McConnell, a Stanford University law professor and a former federal appeals court judge, agreed that the program is potentially “dangerous,” but he called the case that it is illegal “extremely weak.” He likened the NSA’s collection of general data, such as who emailed whom, to an airport search of all passengers.