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“The metadata available is now so fine-grained that it reveals where we’re going, what we’re doing, what our preferences and beliefs might be and who our friends are,” he said.

Federal law and rulings by federal courts have consistently held that metadata, including information about the location of mobile phones, is not covered by the warrant requirements of the U.S. Constitution.

“Unfortunately, technology and the opportunities it presents for surveillance have outpaced our understanding of the Fourth Amendment,” Mr. Wicker said, citing the constitutional ban on unlawful searches and seizures.

Mr. Binney said that, in any case, the NSA already is collecting the content of calls and emails, as well as metadata.

In 2003, according to sworn testimony by a former AT&T engineer, the NSA began building a special room at the company’s switching center in San Francisco and at other AT&T switching centers around the country. Equipment in the room enabled NSA to siphon off a copy of every byte of data running through AT&T’s fiber-optic cable network, according to privacy advocates.