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 Post found nearly 900 additional e-mail addresses, unmasked in the files, that could be strongly linked to U.S. citizens or U.S.residents.

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 Post will not describe in detail, to avoid interfering with 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…

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…

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 other people wrote.

“1 target, 38 others on there,” one analyst wrote. She collected data on them all.