WASHINGTON - Ordinary Internet users, American and non-American alike, far outnumber legally targeted foreigners in the communications intercepted by the National Security Agency from U.S. digital networks, according to a four-month investigation by the Washington Post.

Nine of 10 account holders found in a large cache of intercepted conversations, which former NSA contractor Edward Snowden provided in full to the Washington Post, were not the intended surveillance targets but were caught in a net the agency had cast for somebody else.

Many of them were Americans. Nearly half of the surveillance files, a strikingly high proportion, contained names, e-mail addresses, or other details that the NSA marked as belonging to U.S. citizens or residents. NSA analysts masked, or "minimized," more than 65,000 such references to protect Americans' privacy, but the Washington Post found nearly 900 additional e-mail addresses, unmasked in the files, that could be strongly linked to U.S. citizens or residents.

Surveillance files

The surveillance files highlight a policy dilemma that has been aired only abstractly in public. There are discoveries of considerable intelligence value in the intercepted messages - and collateral harm to privacy on a scale that the Obama administration has not been willing to address.

Among the most valuable contents - which the Washington Post will not describe in detail, to avoid harm to ongoing operations - are fresh revelations about a secret overseas nuclear project, double-dealing by an ostensible ally, a military calamity that befell an unfriendly power, and the identities of aggressive intruders into U.S. computer networks.

Months of tracking communications across more than 50 alias accounts, the files show, led directly to the 2011 capture in Abbottabad of Muhammad Tahir Shahzad, a Pakistan-based bomb builder, and Umar Patek, a suspect in a 2002 terrorist bombing on the Indonesian island of Bali. At the request of CIA officials, the Post is withholding other examples that officials said would compromise operations.

Voyeuristic quality

Many other files, described as useless by the analysts but nonetheless retained, have a startlingly intimate, even voyeuristic quality. They tell stories of love and heartbreak, illicit sexual liaisons, mental-health crises, political and religious conversions, financial anxieties, and disappointed hopes. The daily lives of more than 10,000 account holders who were not targeted are catalogued and recorded nevertheless.

In order to allow time for analysis and outside reporting, neither Snowden nor the Washington Post has disclosed until now that he obtained and shared the content of intercepted communications. The cache Snowden provided came from domestic NSA operations under the broad authority granted by Congress in 2008 with amendments to the Foreign Intelligence Surveillance Act. FISA content is generally stored in closely controlled data repositories, and for more than a year, senior government officials have depicted it as beyond Snowden's reach.

The Washington Post reviewed roughly 160,000 intercepted e-mail and instant-message conversations, some of them hundreds of pages long, and 7,900 documents taken from more than 11,000 online accounts.

The material spans President Obama's first term, from 2009 to 2012, a period of exponential growth for the NSA's domestic collection.

Taken together, the files offer an unprecedented vantage point on the changes wrought by Section 702 of the FISA amendments, which enabled the NSA to make freer use of methods that for 30 years had required probable cause and a warrant from a judge. One program, code named PRISM, extracts content stored in user accounts at Yahoo, Microsoft, Facebook, Google, and five other leading Internet companies. Another, known inside the NSA as Upstream, intercepts data on the move as it crosses the U.S. junctions of global voice and data networks.

No government oversight body, including the Justice Department, the Foreign Intelligence Surveillance Court, intelligence committees in Congress or the president's Privacy and Civil Liberties Oversight Board, has delved into a comparably large sample of what the NSA actually collects - not only from its targets but from people who may cross a target's path.

Among the latter are medical records sent from one family member to another, resumes from job hunters, and academic transcripts of schoolchildren. In one photo, a young girl in religious dress beams at a camera outside a mosque.