For the first time since implementing radio technology in the early 20th century, the San Francisco Police Department has begun encrypting practically all of its radio transmissions, making them inaccessible to the public.

SFPD confirmed the move to BuzzFeed News, explaining it had rolled out a new encryption protocol on Monday in compliance with a California Department of Justice mandate to protect personally identifiable information (PII).

Police scanner hobbyists were the first to clock the change. Desmond Crisis, a technologist based in San Francisco, noticed this week that the police scanners he usually listens to had gone silent. Crisis likes to monitor local police channels for nearby incidents and threats to public safety. But now, the majority of police transmissions have disappeared behind a wall of encryption.

“Effectively all police department traffic will be encrypted,” Adam Lobsinger, an SFPD spokesperson, told BuzzFeed News on Thursday. “Almost no police radio traffic will be heard, and what is left unencrypted is going to be on the Department of Emergency Management side.”

The city’s Department of Emergency Management (DEM), which routes 911 calls to police or fire units, is also developing a protocol for encrypting certain radio transmissions, though some DEM dispatches may remain unencrypted. Communications between SFPD and DEM could previously be heard on scanners; now, the public is no longer privy to the law enforcement side of those exchanges. The DEM has not yet responded to a request for comment.

SFPD’s use of widespread encryption has barred hobbyists like Crisis — as well as journalists and audio platforms that broadcast police streams — from monitoring even standard police operations. It is part of a growing trend among US law enforcement that worries government transparency advocates about the consequences of a less visible police force. Police in Illinois, Minnesota, Virginia, and other states have recently encrypted their radio communications to some degree. Privacy law experts say that blanket encryption is an extreme response to perceived threats, and that police radio traffic is a necessary public resource.

“These radio transmissions are an important window the public has into what police do,” David Snyder, executive director of the First Amendment Coalition, told BuzzFeed News.

SFPD previously framed the coming encryption of its radio feeds as a partial measure, and not the effective blocking of all radio traffic. In May, Sgt. Michael Andraychak told Palo Alto’s Daily Post that the select encryption of PII would be “sort of striking a balance.”

The agency stated in a June media advisory that it is “committed to transparency as the law allows and will be fulfilling the public’s interest by not encrypting all of [its] channels.” To limit the transmission of PII, the agency would encrypt portions of radio traffic containing sensitive information, including discussions of sexual assault and domestic violence.

Scanner hobbyists on radio forums and subreddits first noticed the change, with many worrying about its potential to undermine police transparency at a time when distrust in the police is running high. They have been tracking the department’s shift from an analog radio system to a digital one in November. SFPD replaced its 20-year-old system with a newer version that provides wider coverage and clearer audio and conforms to interoperability standards.

“Public safety information is public by nature,” Crisis told BuzzFeed News.

The disappearance of police channels was also noticed by the controversial crime watch app Citizen, which relies on these transmissions to populate its app with breaking incidents. BuzzFeed News confirmed with the company that police dispatches were no longer appearing on the app in the San Francisco area. At the time, a Citizen spokesperson believed it to be a glitch.

“We’ve reached out to law enforcement regarding the glitch in public access to radio transmissions in San Francisco,” the spokesperson said. “We’re looking forward to collaborating with them to ensure that news outlets and safety technology like Citizen keeps people informed about safety in the city. Rapid access to information saves lives and helping make that content relevant, factual and easy to access is core to our mission.”

When asked about transparency concerns, Lobsinger directed BuzzFeed News to the California Department of Justice, which he said authored the new encryption rules. The department has not yet responded to a request for comment.

In the past year, several other California police departments have also encrypted their radio transmissions in accordance with the DOJ mandate. The statewide rule gave law enforcement agencies two options: fully encrypt their radio channels or selectively encrypt the dissemination of PII. (In California, PII is defined as an individual’s first name or initial and last name in combination with other data, such as a Social Security or driver’s license number.)

SFPD is the second-largest police department in California, reporting 2,100 employees in December, and appears to be the largest in the state to have encrypted all of its radio traffic. (The LAPD, California’s largest police department, appears to only partially encrypt sensitive communications.)

Among California law enforcement, the move to encrypted radio traffic has been staggered. The Daily Post reported in July that some agencies had struggled to implement the new policy change, citing issues like a lack of compatible radio equipment. Police in other cities, however, applied the encryption mandate swiftly and without notice. The Palo Alto Police Department began encrypting its radio channels in January, notifying only the news media and without public input. In April, Palo Alto residents criticized the new lack of transparency after a police officer sicced his service dog on a man wrongly identified as a kidnapping suspect.

Experts question the ultimate outcome of police radio encryption and who it actually protects. Jake Wiener, a domestic surveillance law fellow at EPIC, likened police transmissions to the public’s right to record and broadcast law enforcement activities, noting that documentation of police activity on smartphones has been a powerful driver of anti–police violence movements.

“The majority of information conveyed over police scanners is about the police,” Wiener told BuzzFeed News. “It’s much more of a roadblock to police accountability and community defense than any meaningful privacy protection.”