If you use a cellphone and live in one of these 20 states, there’s a decent chance police have spied on you using a secretive mass surveillance tool called a stingray. But good luck finding out. Because if there’s one thing we know for sure about these devices, it’s that the federal government is fighting tooth and nail to stop you from ever learning anything about them.

Stingrays (also known as cell-site simulators, IMSI catchers and dirtboxes) are devices that identify and track cellphones en masse by acting like fake cell towers, fooling all nearby phones into connecting to them. Last year documents obtained through the Freedom of Information Act revealed that the Federal Bureau of Investigation is requiring state and local police departments to sign nondisclosure agreements before obtaining the devices. But the details of those secret agreements were always completely redacted.

That is until earlier this week, when the ACLU released new stingray documents, including an agreement between the FBI and police in Erie County, New York. It confirms what privacy advocates have suspected: The federal government is intervening at the state and local level to hide information about stingrays at any cost — even when it means withholding evidence or dropping criminal cases.

The agreement bars the Erie County Sheriff’s Office from making any public statements about stingrays and says it must call the FBI to intervene whenever a public records request or court order compels the county to reveal any information about the technology. The accord even says the department must be willing to drop cases, explicitly directing police to “seek dismissal of the case in lieu of using or providing or allowing others to use or provide any information concerning [stingrays]” at the FBI’s request.

That’s right: The FBI is commanding local cops to ignore court orders and sabotage criminal cases rather than reveal information about stingrays.

Other police departments are apparently under the same mandate. In incredible testimony on Wednesday, a Baltimore police detective admitted in court that the FBI has instructed the city’s police to disobey court orders that seek information about stingrays, which he said the department has used 4,300 times since 2007. In a previous case, Baltimore prosecutors threw out evidence after a judge threatened to hold an officer in contempt for refusing to explain how a stingray was used to catch the defendant. And in Florida, a defendant was let off with six months of probation for armed robbery — an offense normally carrying a minimum sentence of four years — again so that police wouldn’t have to tell the court how they used a stingray in the bust.

These are just a few Kafkaesque examples of the extreme measures police have taken to prevent the public from finding out about stingrays. Last year documents requested by the ACLU of Florida were seized by U.S. marshals at the FBI’s behest just hours before they were scheduled to be released under a court order.