The roots of SOP 303 can be traced to the 2005 London subway attacks, in which suicide bombers used their cellphones to detonate underground explosives. Immediately after that attack, U.S. authorities disabled cellular service in New York City’s Hudson River commuter tunnels for almost two weeks. This unprecedented shutdown was made without notice to the public or even wireless carriers. The result, by the DHS’ admission was, “disorder for both government and the private sector at a time when use of the communications infrastructure was most needed.”

Using the argument that releasing any information about SOP 303 would risk public safety, the DHS prevailed in an appeals court after a lower court ruled in favor of making the policy public. In March of this year, however, EPIC’s petition for a rehearing of that appellate court decision was allowed to proceed, with the court issuing an April 27 deadline for the government to respond.

“We have no clue what’s in it or what it’s about,” says Harold Feld, the senior vice president of Public Knowledge, a public interest advocacy group. In 2012 the Electronic Privacy Information Center (EPIC) filed suit in federal court seeking disclosure of information about SOP 303’s basic guidelines and policy procedures.

Adopted without public notice or debate, Standard Operating Procedure (SOP) 303, often referred to as the cellphone kill switch, has been shrouded in secrecy from its inception and has outraged some civil liberties groups battling to make the policy public.

For nearly a decade, the U.S. Department of Homeland Security (DHS) has maintained a policy for unilaterally shutting down private cellular service, over an entire metropolitan area if necessary, in the event of a national crisis.

If signed by the governor, the crime-deterrent switches will be mandatory on devices sold in the state after July 2015

Seeking a more coordinated response, in May 2006 the department’s National Security Telecommunications Advisory Committee approved a set of protocols governing future cellular network shutdowns.

Precisely who has the authority to initiate a shutdown is unclear. According to one of the few public documents describing any aspect of SOP 303, a shutdown request may come from “state Homeland Security advisers, their designees or representatives of the DHS homeland security operations center.” SOP 303 tasks a DHS subagency, the National Coordinating Center for Communications, with asking “a series of questions to determine if the shutdown is a necessary action” before notifying the affected cellular carriers. Just what those questions are remains a secret.

Critics of SOP 303 argue that cellular communication is simply too vital to the public, particularly in times of emergency, to be shut off. Especially if the guidelines for doing so remain secret. “I don’t see any situation where you want to shut down the [cellular] phone network,” says Feld. “In the years since 9/11 we have moved all our critical public safety services onto the cellular network.”

Others question the counterterrorism benefits of shutting down cell service, pointing out that in previous attacks, bombers have detonated devices using their phones’ alarm clock feature, a method that does not require a cellular connection.

Civil liberties groups fear that such far-reaching and unchecked power could be used simply to quell dissent. That’s exactly what happened in August 2011 when officials of Northern California’s Bay Area Rapid Transit system utilized its kill switch to temporarily shut off cellphone service in several subway stations. The shutdown wasn’t in response to a terrorist threat. Its purpose was to prevent a demonstration that organizers planned to hold in those stations protesting a fatal subway shooting by a BART police officer.

“We want the guidelines to be released,” says Alan Butler, an EPIC lawyer who has been involved with the court case from the start. “We want information about the scope of the rules and the questions [DHS] would ask before deactivating service.” Noting that in the BART shutdown there was neither a public threat nor a request to cellular providers to suspend service — BART engineers shut off underground signal amplifiers — he wonders what protocols were followed. “[DHS] is supposed to be coordinating [these] functions. If that didn’t happen here … if there was a procedure in place and it wasn’t used, the question is, Why not?”

In response to the government’s claim that disclosure could allow a bad actor to insert himself in the shutdown process, Butler says, “What we’re asking for is really high-level stuff. We’re not asking for detailed information about how [SOP 303] works … but about the rationale and the policy guidelines.”

“Understanding a policy should not compromise national security,” agrees Feld.

Asked to explain the decision to withhold even basic procedural information about SOP 303 from the public, a DHS representative replied, “We have no comment on this.”