A secretive surveillance company has been quietly selling hidden cameras to police departments and federal agencies, according to records obtained by the watchdog nonprofit MuckRock.

The Special Services Group sales brochure advertises surveillance devices hidden in rocks and trees, as well as a "tombstone cam."

The company has sold its products to dozens of US agencies, including the FBI, DEA, and ICE.

But Special Services Group has gone to great lengths to keep its products secret, even threatening to sue journalists at Vice for reporting on its sales brochure earlier this week.

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A shadowy company with a mission of "Constant Vigilance" is selling hidden cameras and listening devices to government agencies that are disguised as rocks, trees, tombstones, vacuum cleaners, and even baby car seats.

The company, Special Services Group, keeps its gadgets secret — it doesn't list any products on its website, citing "the critical missions of our customers." The hidden camera devices remained unknown to the public until the company's sales pamphlet was published by the transparency nonprofit MuckRock earlier this week.

The pamphlet shines a new light on the mostly unknown tools that law enforcement and federal agencies use to keep tabs on people and track potential suspects. While government agencies routinely publish financial disclosures showing their contracts with companies like Special Services Group, the surveillance devices themselves are generally shrouded in secret.

MuckRock obtained the documents through an open records request filed to the city of Irvine, California. Special Services Group had emailed its "Black Book" brochure of products to the city's police department, which made the brochure public in response to MuckRock's inquiry.

But Special Services Group has since taken great lengths to keep its "Black Book" a secret. The company's lawyers reportedly threatened legal action against journalists at Vice's tech news site, Motherboard, for reporting on the already-public pamphlet, arguing that "the release of the information could result in very serious jeopardy to the lives of law enforcement."

Special Services Group did not immediately respond to Business Insider's request for comment. According to MuckRock, the city of Irvine released the "Black Book" pamphlet after the city's lawyers determined that it was safe to publish and that doing so served the "public interest."

Little more is known about Special Services Group. The company has sold its products to dozens of US agencies, including the FBI, DEA, Secret Service, and ICE, according to public procurement data. Its federal contracts are worth more than $2.5 million in total.

Here are some of the hidden surveillance devices from the "Black Book" pamphlet the company wanted to keep secret.