The US Marshals Service spent more than $10m on secret, possibly airborne equipment and software for warrantless surveillance of Americans’ cellphones, documents obtained by the American Civil Liberties Union (ACLU) have revealed.

The documents are heavily redacted, but they show that the Marshals Service purchased more than $10m in hardware and software from Harris Corporation, the manufacturer of the cellphone snooping device known as a Stingray, between 2009 and 2014.

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Stingrays are one of a class of sophisticated devices collectively called “cell-site simulators”. They work by pretending to be a cellphone tower to gather metadata, location information, and in some cases content from phones that connect to them.

Despite the efforts of some lawmakers, including Republican representative Jason Chaffetz of Utah and Democratic senator Ron Wyden of Oregon, Stingray use does not currently require a search warrant; merely an antiquated court order called a “trap-and-trace”, which was designed for obtaining phone records, not content, and represents a low judicial bar when used for devices as powerful as Stingray.

The scope of Stingray use is not fully known, as the devices are cloaked in extreme secrecy. In 2015, a Guardian investigation uncovered a non-disclosure agreement which local police departments were forced to sign with the FBI before using the devices, which went so far as to mandate local prosecutors to throw cases, rather than reveal in open court that a Stingray had been used.

But over the past year, investigations by the Guardian, USA Today and Ars Technica, among other outlets, as well as the ACLU, have brought the number of known police departments and federal agencies to use the devices to 61.

Roughly the size of a suitcase, the devices can be mounted in a car or, as the documents hint at, in planes. An investigation in 2014 by the Wall Street Journal found that the Marshals Service was also using similar technology, known as a Dirtbox, installed in a fleet of Cessna light aircraft.

Documents released earlier in March by the Electronic Frontier Foundation following a similar freedom of information request revealed that the FBI has also been mounting the devices in aircraft, with no overarching policy or guidelines to their use by agents.

It is unclear, according to Nate Wessler, a staff attorney at the ACLU, how much of the $10m in Harris Corporation equipment the agency purchase is being aircraft mounted. But, Wessler said, the cache of documents released today were obtained following requests to several federal agencies under the freedom of information act which specifically asked about aircraft-mounted surveillance equipment.

Airborne use of cell-site simulators is less frequent than ground-vehicle mounted use, partly because many of the agencies using the devices are local police departments without the resources a federal agency like the Marshals or the FBI can bring to bear.

Despite this, for Wessler, the privacy implications for bystanders of airborne Stingray use is “by definition greater, because when you’re flying around with one of these devices it can cover a pretty wide geographic area.”

The Drug Enforcement Administration’s El Paso division spent $400,000 on cell-site simulator devices, the ACLU documents revealed. The DEA was already known to use Stingrays, but Wessler said that it was clear from their response letter that this particular purchase was specifically intended for airborne use.

Representatives for the US Marshals Service and Harris Corporation declined to comment for this story.