For at least six years, law enforcement officials working on a counter narcotics program have had routine access, using subpoenas, to an enormous AT&T database that contains the records of decades of Americans' phone calls — parallel to but covering a far longer time than the National Security Agency's hotly disputed collection of phone call logs.
The Hemisphere Project, a partnership between federal and local drug officials and AT&T that has not previously been reported, involves an extremely close association between the government and the telecommunications giant.
The government pays AT&T to place its employees in drug-fighting units around the country. Those employees sit alongside Drug Enforcement Administration agents and local detectives and supply them with the phone data from as far back as 1987.
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The project comes to light at a time of vigorous public debate over the proper limits on government surveillance and on the relationship between government agencies and communications companies. It offers the most significant look to date at the use of such large-scale data for law enforcement, rather than for national security.
The scale and longevity of the data storage appears to be unmatched by other government programs, including the N.S.A.'s gathering of phone call logs under the Patriot Act. The N.S.A. stores the data for nearly all calls in the United States, including phone numbers and time and duration of calls, for five years.
Hemisphere covers every call that passes through an AT&T switch — not just those made by AT&T customers — and includes calls dating back 26 years, according to Hemisphere training slides bearing the logo of the White House Office of National Drug Control Policy. Some four billion call records are added to the database every day, the slides say; technical specialists say a single call may generate more than one record. Unlike the N.S.A. data, the Hemisphere data includes information on the locations of callers.
The slides were given to The New York Times by Drew Hendricks, a peace activist in Port Hadlock, Wash. He said he had received the PowerPoint presentation, which is unclassified but marked "Law enforcement sensitive," in response to a series of public information requests to West Coast police agencies.
The program was started in 2007, according to the slides, and has been carried out in great secrecy.
"All requestors are instructed to never refer to Hemisphere in any official document," one slide says. A search of the Nexis database found no reference to the program in news reports or Congressional hearings.
The Obama administration acknowledged the extraordinary scale of the Hemisphere database and the unusual embedding of AT&T employees in government drug units in three states.
But they said the project, which has proved especially useful in finding criminals who discard cellphones frequently to thwart government tracking, employed routine investigative procedures used in criminal cases for decades and posed no novel privacy issues.
Crucially, they said, the phone data is stored by AT&T, and not by the government as in the N.S.A. program. It is queried for phone numbers of interest mainly using what are called "administrative subpoenas," those issued not by a grand jury or a judge but by a federal agency, in this case the D.E.A.
Brian Fallon, a Justice Department spokesman, said in a statement that "subpoenaing drug dealers' phone records is a bread-and-butter tactic in the course of criminal investigations."
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Mr. Fallon said that "the records are maintained at all times by the phone company, not the government," and that Hemisphere "simply streamlines the process of serving the subpoena to the phone company so law enforcement can quickly keep up with drug dealers when they switch phone numbers to try to avoid detection."
He said that the program was paid for by the D.E.A. and the White House drug policy office but that the cost was not immediately available.
Officials said four AT&T employees are now working in what is called the High Intensity Drug Trafficking Area program, which brings together D.E.A. and local investigators — two in the program's Atlanta office and one each in Houston and Los Angeles.
Daniel C. Richman, a law professor at Columbia, said he sympathized with the government's argument that it needs such voluminous data to catch criminals in the era of disposable cellphones.
"Is this a massive change in the way the government operates? No," said Mr. Richman, who worked as a federal drug prosecutor in Manhattan in the early 1990s. "Actually you could say that it's a desperate effort by the government to catch up."
But Mr. Richman said the program at least touched on an unresolved Fourth Amendment question: whether mere government possession of huge amounts of private data, rather than its actual use, may trespass on the amendment's requirement that searches be "reasonable." Even though the data resides with AT&T, the deep interest and involvement of the government in its storage may raise constitutional issues, he said.
Jameel Jaffer, deputy legal director of the American Civil Liberties Union, said the 27-slide PowerPoint presentation, evidently updated this year to train AT&T employees for the program, "certainly raises profound privacy concerns."
"I'd speculate that one reason for the secrecy of the program is that it would be very hard to justify it to the public or the courts," he said.
Mr. Jaffer said that while the database remained in AT&T's possession, "the integration of government agents into the process means there are serious Fourth Amendment concerns."
Mr. Hendricks filed the public records requests while assisting other activists who have filed a federal lawsuit saying that a civilian intelligence analyst at an Army base near Tacoma infiltrated and spied on antiwar groups. (Federal officials confirmed that the slides are authentic.)
Mark A. Siegel, a spokesman for AT&T, declined to answer more than a dozen detailed questions, including ones about what percentage of phone calls made in the United States were covered by Hemisphere, the size of the Hemisphere database, whether the AT&T employees working on Hemisphere had security clearances and whether the company has conducted any legal review of the program
"While we cannot comment on any particular matter, we, like all other companies, must respond to valid subpoenas issued by law enforcement," Mr. Siegel wrote in an e-mail.
Representatives from Verizon, Sprint and T-Mobile all declined to comment on Sunday in response to questions about whether their companies were aware of Hemisphere or participated in that program or similar ones. A federal law enforcement official said that the Hemisphere Project was "singular" and that he knew of no comparable program involving other phone companies.
The PowerPoint slides outline several "success stories" highlighting the program's achievements and showing that it is used in investigating a range of crimes, not just drug violations. The slides emphasize the program's value in tracing suspects who use replacement phones, sometimes called "burner" phones, who switch phone numbers or who are otherwise difficult to locate or identify.
In March 2013, for instance, Hemisphere found the new phone number and location of a man who impersonated a general at a San Diego Navy base and then ran over a Navy intelligence agent. A month earlier the program helped catch a South Carolina woman who had made a series of bomb threats.
And in Seattle in 2011, the document says, Hemisphere tracked drug dealers who were rotating prepaid phones, leading to the seizure of 136 kilos of cocaine and $2.2 million.