Local law enforcement agencies from suburban Southern California to rural North Carolina have been using an obscure cellphone tracking tool, at times without search warrants, that gives them the power to follow people’s movements months back in time, according to public records and internal emails obtained by The Associated Press.

Police have used “Fog Reveal” to search hundreds of billions of records from 250 million mobile devices, and harnessed the data to create location analyses known among law enforcement as “patterns of life,” according to thousands of pages of records about the company.

Sold by Virginia-based Fog Data Science LLC, Fog Reveal has been used since at least 2018 in criminal investigations ranging from the murder of a nurse in Arkansas to tracing the movements of a potential participant in the Jan. 6 insurrection at the Capitol. The tool is rarely, if ever, mentioned in court records, something that defense attorneys say makes it harder for them to properly defend their clients in cases in which the technology was used.

The company was developed by two former high-ranking Department of Homeland Security officials under former President George W. Bush. It relies on advertising identification numbers, which Fog officials say are culled from popular cellphone apps such as Waze, Starbucks and hundreds of others that target ads based on a person’s movements and interests, according to police emails. That information is then sold to companies like Fog.

“It’s sort of a mass surveillance program on a budget,” said Bennett Cyphers, a special adviser at the Electronic Frontier Foundation, a digital privacy rights advocacy group.

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This story, supported by the Pulitzer Center on Crisis Reporting, is part of an ongoing Associated Press series, “Tracked,” that investigates the power and consequences of decisions driven by algorithms on people’s everyday lives.

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The documents and emails were obtained by EFF through Freedom of Information Act requests. The group shared the files with The AP, which independently found that Fog sold its software in about 40 contracts to nearly two dozen agencies, according to GovSpend, a company that keeps tabs on government spending. The records and AP’s reporting provide the first public account of the extensive use of Fog Reveal by local police, according to analysts and legal experts who scrutinize such technologies.

Federal oversight of companies like Fog is an evolving legal landscape. On Monday, the Federal Trade Commission sued a data broker called Kochava that, like Fog, provides its clients with advertising IDs that authorities say can easily be used to find where a mobile device user lives, which violates rules the commission enforces. And there are bills before Congress now that, if passed, would regulate the industry.

“Local law enforcement is at the front lines of trafficking and missing persons cases, yet these departments are often behind in technology adoption,” Matthew Broderick, a Fog managing partner, said in an email. “We fill a gap for underfunded and understaffed departments.”

Because of the secrecy surrounding Fog, however, there are scant details about its use and most law enforcement agencies won’t discuss it, raising concerns among privacy advocates that it violates the Fourth Amendment to the U.S. Constitution, which protects against unreasonable search and seizure.

What distinguishes Fog Reveal from other cellphone location technologies used by police is that it follows the devices through their advertising IDs, unique numbers assigned to each device. These numbers do not contain the name of the phone’s user, but can be traced to homes and workplaces to help police establish pattern-of-life analyses.

“The capability that it had for bringing up just anybody in an area whether they were in public or at home seemed to me to be a very clear violation of the Fourth Amendment,” said Davin Hall, a former crime data analysis supervisor for the Greensboro, North Carolina, Police Department. “I just feel angry and betrayed and lied to.”

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Hall resigned in late 2020 after months of voicing concerns about the department’s use of Fog to police attorneys and the city council.

While Greensboro officials acknowledged Fog’s use and initially defended it, the police department said it allowed its subscription to expire earlier this year because it didn’t “independently benefit investigations.”

But federal, state and local police agencies around the U.S. continue to use Fog with very little public accountability. Local police agencies have been enticed by Fog’s affordable price: It can start as low as $7,500 a year. And some departments that license it have shared access with other nearby law enforcement agencies, the emails show.

Police departments also like how quickly they can access detailed location information from Fog. Geofence warrants, which tap into GPS and other sources to track a device, are accessed by obtaining such data from companies, like Google or Apple. This requires police to obtain a warrant and ask the tech companies for the specific data they want, which can take days or weeks.

Using Fog’s data, which the company claims is anonymized, police can geofence an area or search by a specific device’s ad ID numbers, according to a user agreement obtained by AP. But, Fog maintains that “we have no way of linking signals back to a specific device or owner,” according to a sales representative who emailed the California Highway Patrol in 2018, after a lieutenant asked whether the tool could be legally used.

Despite such privacy assurances, the records show that law enforcement can use Fog’s data as a clue to find identifying information. “There is no (personal information) linked to the (ad ID),” wrote a Missouri official about Fog in 2019. “But if we are good at what we do, we should be able to figure out the owner.”

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Fog’s Broderick said in an email that the company does not have access to people’s personal information, and draws from “commercially available data without restrictions to use,” from data brokers “that legitimately purchase data from apps in accordance with their legal agreements.” The company refused to share information about how many police agencies it works with.

“We are confident Law Enforcement has the responsible leadership, constraints, and political guidance at the municipal, state, and federal level to ensure that any law enforcement tool and method is appropriately used in accordance with the laws in their respective jurisdictions,” Broderick said.

Prior to publication, Fog’s Broderick refused to say how the company got data from Starbucks and Waze. But on Thursday, he said he did not know how data aggregators collected the information Fog Reveal draws from, or the specific apps from which the data was drawn.

“Search warrants are not required for the use of the public data,” he said Thursday, adding that the data his product offers law enforcement is “lead data” and should not be used to establish probable cause.

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AP National Writer Allen G. Breed contributed from Greensboro, North Carolina. Dearen reported from New York and Burke reported from San Francisco.

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This reporting was produced in collaboration with researchers Janine Graham, Nicole Waddick and Jane Yang as well as the University of California, Berkeley’s Human Rights Center Investigations Lab and School of Law.

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Follow Garance Burke and Jason Dearen on Twitter at @garanceburke and @jhdearen. Contact AP’s global investigative team at Investigative@ap.org or https://www.ap.org/tips/