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 in 10 account holders found in a large cache of intercepted conversations, which former NSA contractor Edward Snowden provided in full to The 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, email 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 email addresses, unmasked in the files, that could be strongly linked to U.S. citizens or residents.The surveillance files highlight a policy dilemma that has been aired only abstractly in public. The intercepted messages include discoveries of considerable intelligence value — as well as collateral harm to privacy on a scale that the Obama administration has not been willing to address.Among the most valuable contents — which The Post will not detail to avoid harming 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 the 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.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 cataloged and recorded nevertheless.To allow time for analysis and outside reporting, neither Snowden nor The Post has disclosed until now that he obtained and shared the content of intercepted communications. The cache he 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 Post reviewed roughly 160,000 intercepted email and instant-message conversations, some of them hundreds of pages, and 7,900 documents taken from more than 11,000 online accounts.The material spans President Barack 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 also from people who may cross a target’s path.Medical recordsIncluded are medical records sent from one relative to another, résumés from job hunters and academic transcripts of schoolchildren. In one photo, a girl in religious dress beams at a camera outside a mosque.Scores of pictures show infants and toddlers in bathtubs, on swings, sprawled on their backs and kissed by their mothers. In some photos, men show off their physiques. In others, women model lingerie, leaning suggestively into a webcam or striking risque poses in shorts and bikini tops.“None of the hits that were received were relevant,” two Navy cryptologic technicians write in one of many summaries of nonproductive surveillance. “No additional information,” a civilian analyst writes. Another makes fun of a suspected kidnapper, newly arrived in Syria before the current civil war, who begs for employment as a janitor and makes wide-eyed observations about the state of undress displayed by women on local beaches.By law, the NSA may “target” only foreign nationals located overseas unless it obtains a warrant based on probable cause from a special surveillance court. For collection under PRISM and Upstream rules, analysts must state a reasonable belief that the target has information of value about a foreign government, a terrorist organization or the spread of nonconventional weapons.Most people caught up in those programs are not the targets and would not lawfully qualify as such. “Incidental collection” of third-party communications is inevitable in many forms of surveillance. But in other contexts, the U.S. government works harder to limit and discard irrelevant data. In criminal wiretaps, for example, the FBI is supposed to stop listening to a call if a suspect’s wife or child is using the phone.People can be swept up in surveillance aimed at a valid foreign target in many ways. Some of those in the Snowden archive were monitored because they interacted directly with a target, but others had more tenuous links.If a target entered an online chat room, the NSA collected the words and identities of every person who posted there, regardless of subject, as well as every person who simply “lurked,” reading passively what others wrote.“1 target, 38 others on there,” one analyst wrote. She collected data on them all.In other cases, the NSA designated as its target the Internet protocol, or IP, address of a computer server used by hundreds of people.