The U.S. Marshals Service spent more than $10 million on secret spying equipment known as Stingrays to conduct mass surveillance operations—possibly from the air—on Americans' cell phones, according to documents published Thursday by the ACLU.

The heavily redacted documents reveal that the Marshals purchased more than $10 million in software and hardware between 2009 and 2014 from the Harris Corporation, the leading U.S. manufacturer of Stingrays, the controversial surveillance devices that gather metadata by simulating cell phone towers and tricking nearby devices into connecting with them. The ACLU acquired the documents through a Freedom of Information Act (FOIA) request.

Although the names of specific items purchased are redacted, the files state that they are needed "to aid in the apprehension of fugitives" and "are relatively low in power consumption and can be operated via standard automotive 12V DC or standard 110V," meaning that they are mobile and can be installed in vehicles, said ACLU staff attorney Nathan Freed Wessler.

ACLU filed the FOIA following a Wall Street Journal investigation, published in 2014, which found that the Marshals Service has been using Stingrays in the air since 2007.

Stingrays already raise privacy concerns "because they can sweep up information about bystanders’ phones and precisely locate people, including inside their homes," Wessler writes in a blog post. "Flying a Stingray over a major metropolitan area magnifies the concern by subjecting potentially large numbers of bystanders to privacy violations."

He continues:

Oddly, the Marshals Service provided only records of purchases from the Harris Corporation, with no records of purchases from Digital Receiver Technology (DRT). According to The Wall Street Journal, the marshals use DRT’s technology, known as “DRTboxes” or “dirtboxes,” for airborne operations. This strongly suggests that the Marshals Service withheld relevant records from their response.

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Among the documents are a set of policy guidelines that does not place explicit limits on the airborne surveillance using the simulators.

Law enforcement are not required to obtain a search warrant before deploying Stingrays in investigations, only a pen register, which needs a lower standard of probable cause to collect phone data.

A number of recent investigations and disclosures have revealed that police departments throughout the country that have Stingrays are using them to conduct investigations—and often as a first resort. In one case, New York homicide police appeared to use the device to track down a witness, rather than a suspect.

The documents were released just a day after 45 civil rights and public interest groups published a letter (pdf) and delivered more than 34,500 petitions to the Federal Communications Commission (FCC) and the Department of Justice (DOJ), demanding that the agencies investigate and regulate the use of Stingrays by law enforcement.

"News story after story has broken regarding the use of stingray devices and other cell-site simulators for domestic surveillance by law enforcement agencies," said Emily Hong, policy program associate at New America's Open Technology Institute, one of the organizations that signed the letter. "But the technology’s widespread adoption has unfortunately not brought any more transparency or accountability to the devices’ use."

Rashad Robinson, executive director of Color of Change, another signatory, stated, "Law enforcement's warrantless and reckless use of this invasive surveillance technology is completely unacceptable. We know all too well that unrestricted surveillance power gets disproportionately used against Black communities."

"In light of the widespread and illegal use of this surveillance technology and law enforcement’s long history of surveilling Black communities, we are calling on Chairman Tom Wheeler and the FCC to use their authority over Stingray devices to suspend the warrantless use of these devices and require all law enforcement agencies to publicly register their devices and publicly share their policies governing the use of this device," Robinson said.