While protesters across the United States have to battle law enforcement’s use of rubber bullets and tear gas at demonstrations, the biggest danger they face may be in their pockets.

Anyone attending a protest or entering an area near one can be vulnerable to digital surveillance by law enforcement through their phones. Now, activists and organizations leading demonstrations are warning everyone attending to take necessary precautions to ensure the safety of personal data stored on phones while protesting.

Law enforcement agencies in the United States have used digital surveillance to track protesters for several years. Documents on the protests that took place at the University of California, Berkeley in 2017 proved local police enlisted phone providers to assist the police in identifying the names and phone numbers of demonstrators.

“In the United States, and across the world, any protester who brings a phone to a public demonstration is tracked and that person’s presence at the event is duly recorded in commercial datasets,” Charlie Warzel and Stuart Thompson of The New York Times said. “Like the rest of us, they are only as secure as the least secure apps on their phones.”

There are two primary ways in which law enforcement can infiltrate protesters’ personal data from the phones. The first involves the police physically confiscating a phone from a protester after they detain or arrest them. The second, and more invasive, way police can breech personal data is through the use of law enforcement surveillance. This includes the interception of text conversations and the utilization of tracking technology such as facial recognition and license plate scanners.

Geofence warrants have become one of the most popular tools used by local law enforcement officials in recent years. Sometimes referred to as reverse location searches, this process involves police soliciting the help of tech companies to gather information stored on mobile devices in a specific area of interest.

This process is commonly used when local law enforcement is looking to solve a crime and want more information on the people that were at or around the scene to search for suspicious activity. If the tech company agrees to provide law enforcement with the information, then the personal data of anyone within the vicinity of a crime could be leaked to local police departments.

The tech company that receives the most requests is Google due to a feature that records the location history of the user. In 2019, a report from The New York Times revealed Google regularly receives hundreds of requests over the course of a week.

“We vigorously protect the privacy of our users while supporting the important work of law enforcement,” Richard Salgado, director of law enforcement and information security at Google, said in a statement. “We developed a process specifically for these requests that is designed to honor our legal obligations while narrowing the scope of data disclosed.”

Law enforcement could very well obtain access to protesters’ personal data on their phones through geofence warrant requests if they are investigating a crime that took place at or near a demonstration. With the number of techniques and tools law enforcement have at their disposal to track protesters at demonstrations, many activist organizations and media outlets have created extensive guides for people to retain their privacy while showing support.

“The device in your pocket is definitely going to give off information that could be used to identify you,” Harlo Holmes, newsroom security director for the Freedom of the Press Foundation, said.

This major risk is why many experts advise demonstrators to attend protests without their personal phone altogether. If someone attending a protest knows they may need a phone for an emergency, they should either bring a burner phone that does not have their personal information or have their primary phone off if they can.

Phone calls and text messages can also be vulnerable and exploited through law enforcement surveillance. Both end-to-end encryption and self-deleting messages in programs such as WhatsApp can ensure your conversations are not easily accessed by police.

Moreover, a strong passcode can be imperative to restricting law enforcement from gaining access to a phone during a protest. While unlocking a phone with a fingerprint or face recognition can be more convenient, it can easily be exploited by a law enforcement officer if it is confiscated.

Taking safety precautions to bolster digital security prior to attending a demonstration is vital for any protester looking to stay safe and secure while making their voice heard.

“It’s clear the government is bringing the full force of the surveillance state to monitor these uprisings,” Evan Greer, director for Massachusetts non-profit Fight for the Future, wrote in a Twitter post about digital security at protests. “Remember that taking these steps isn’t just about protecting yourself, it’s about protecting others who may be more at risk than you because they are undocumented, have a criminal record, [or] have an underlying health condition that would make an arrest life threatening.”