At the Caravan for Justice’s first stop, Patrisse Cullors took the mic halfway through the rally she’s helped organize. “Raise your hand if you’ve lost a loved one at the hand of the state,” she said.

Five hands went up. Cullors raised her own. A young woman with asymmetrical, dreaded hair and splatter-painted Birkenstocks, Cullors is a co-founder of Black Lives Matter, a national network of activists that campaigns for the humane and equal treatment of African Americans by law enforcement. “We’re not just here to tell you about the tragedy,” she said. “We’re also here to tell you about things you can do.”

Organizers of the Caravan for Justice kicked off their tour of California on a hot afternoon in Frank Ogawa Plaza. Two-dozen activists in sensible shoes stood in a loose circle under the oak trees. Parked behind them was a 15-seat van. The caravan’s organizers planned to whistle-stop tour their way through Stockton and Salinas, taking shifts as they drove the van to Los Angeles, hitting nine cities in ten days. They planned to host teach-ins on police brutality and introduce the public to Mobile Justice CA, an app that enables users to film law enforcement and send a copy of their footage to the American Civil Liberties Union of California for review. The ACLU of California’s legal intake team will access each video submitted by the app and determine whether or not the officers violated the law. If they did, the ACLU will have grounds to pursue legal action. The app also includes a “witness function,” which when activated broadcasts the user’s GPS location to other app users within a three-mile radius, alerting them to a nearby police action so that they can come bear witness to it themselves.

Created by the ACLU in conjunction with Quadrant 2, the software company of self-described “Apptivist” Jason Van Anden, Mobile Justice CA is one of an evolving collection of apps that enhances citizens’ abilities the track and document their interactions with police. Anden created the app “I’m Getting Arrested” after a friend’s girlfriend was threatened with arrest at Occupy Wall Street. It inspired “Stop and Frisk Watch,” Mobile Justice CA’s predecessor at the New York ACLU, which sends users messages that alert them to nearby police stops, among other functions. There’s CopWatch, which launched in Toronto, and Y-Stop, which launched in London. Following the launch of Mobile Justice CA, ACLU new media strategist Marcus Benigno said that student activists in Hong Kong contacted their office. They want to create an app of their own.

The caravan was jointly organized by the ACLU of California—the state branch of the non-partisan constitutional rights advocacy group—and the Ella Baker Center for Human Rights, a local non-profit that lobbies for criminal justice reform. In addition to co-founding Black Lives Matter, Cullors is the Ella Baker Center’s director of truth and reinvestment, a position through which she organizes people of color to document and legally respond to unfair treatment by law enforcement.

At the caravan’s first stop, activists told stories about people of color they knew who died while in legal custody. “My heart can’t break into any smaller pieces,” said an Ella Baker Center volunteer after listening to them. “The other day, I thought about what people would say about me if I was the next person.”

Cephus and Beatrice Johnson also joined the rally. Their nephew was Oscar Grant, who was fatally shot by BART police officer Johannes Mehserle on New Years Day, 2009. Cephus Johnson said he supports the idea of an app, and that the cellphone videos of his nephew’s death taken by bystanders helped provide evidence that it was unjustified. “There wasn’t one recording that recorded Oscar,” he said. “There were many recordings that recorded Oscar.” He said he planned to download the Mobile Justice CA app himself.

“And I get stopped,” Johnson said. “So when I get stopped don’t worry—I’m going to have my eyes on.”

According to Benigno, Mobile Justice CA surpassed 100,000 downloads in its first two months. Over 160,000 people had downloaded the app as of last week’s count, he said. ACLU branches in six other states have released similar products, and offices in thirteen additional states will launch their own apps on October 23rd. ACLU personnel have promoted the app as “the people’s body camera.”

“I think this app is technology that has the potential to save black lives,” Cullors said.

Mobile Justice CA informs users of their constitutional rights and streamlines their access to legal counsel. According to the app’s proponents, it also addresses issues that prevent people from successfully pursuing legal action against the police. The First Amendment protects citizens’ right to document their interactions with law enforcement, but that doesn’t prevent some officers from confiscating or destroying cameras. According to Benigno, the app is meant to circumvent that problem by transferring a copy of the video to the ACLU’s cloud database.

And while many police officers are required to wear body cameras, in practice they can turn them off at their discretion, says Nate Cardozo, a staff attorney at the Electronic Frontier Foundation, a non-profit organization that defends citizens’ right to free speech and privacy online. “That eliminates a lot of the accountability function related to those cameras,” he said in an interview. The ACLU’s app, he said, is “an interesting and rhetorically powerful attempt to make that accountability something that an officer can’t just choose to turn off.”

To activists and civil rights advocates, it’s clear just how powerful bystander videos of police actions can be. Benigno credits recent video footage of officer involved shootings with galvanizing a national conversation about the use of force by police.

In an interview last week, Cullors said that video can give people who’ve been victimized by the police compelling evidence that corroborates their version of events. “The police are often saying that we have guns, that we struck first,” she said. “And the videos show time and time again that people are running, that people are on their backs, that people are in the car with their hands up.”

According to Benigno, the ACLU has already received 20,000 videos thanks to the app. Mobile Justice CA’s license agreement stipulates that the ACLU’s legal intake team will only look at videos that come with reports, in part to make the workload manageable. “No one’s going to sit there for 10 hours a day watching videos,” Benigno said. Users can submit written reports with their videos from the app’s interface and can file their report by phone if the police confiscate their recording device. Videos are organized by date and location and kept on retention for 60 days in what Benigno describes as a secure, encrypted database. The app doesn’t gather users’ data without their consent, he says, and all data affiliated with the videos that users submit to the ACLU is erased once the 60-day period has elapsed.

Of the 20,000 videos submitted so far, Benigno says that none of them have shown officers behaving illegally or led to legal action. Many users have used the app to film law enforcement during protests or arrests as a preemptive measure in case violence or other legal problems happen. “You’ll hear people on the tape saying, ‘I’ve got the ACLU on my phone! The ACLU’s watching you!’” Benigno said. “It’s empowering, but of course we hope that people do not narrate over what’s going on, so that they allow us to hear the encounter.”

Benigno’s office has noticed small shifts in their users’ behavior when the app is filming. “They narrate,” he said. “They show their faces.”

Users should also remember to keep a safe distance from police actions, he said. The public has the right to film law enforcement, he said, but “we’re not ensuring that you won’t [still] be arrested.”

The app’s users aren’t alone in modifying their behavior in front of the camera. “It has somewhat of a chilling effect on officers’ execution of their duties,” said Bay Area attorney Justin Buffington, who has defended police officers accused of misconduct for the past eight years. He supports civilians’ right to film police encounters and agrees that their videos can be powerful evidence in legal cases. He also says it can be incomplete. “I think it’s important to bear in mind that it’s a tool,” he said in a phone interview. “It’s a snapshot. It’s not a complete indicator of what’s going on.”

Buffington’s past clients include Oakland police officers Hector Jimenez and Robert Roche, both of whom were fired by the Oakland Police Department and then reinstated with back pay under arbitration agreements. Jimenez was fired in 2008 after fatally shooting a suspect of color three times in the back. Roche had shot and killed people of color in the line of duty on three separate occasions. He then gained notoriety in 2011 after shooting a teargas canister into a crowd of Occupy protesters who were trying to help Scott Olsen, an injured activist. The incident was caught on video, which Buffington says made it global news.

“It helps reveal some of the details involved, but not the entire picture,” he said of the footage. The city’s investigation found that Roche didn’t know that the protesters he fired on had converged to help Olsen, said Buffington; according to the attorney, Roche thought they had gathered there for other reasons. Roche was also accused of firing on the protesters at close range, but video footage “is two dimensional,” said Buffington. “So it’s very difficult to gage things like depth—and that’s extremely important in this case.”

According to Buffington, bystanders who film a police action often fail to catch the series of instigating events that preceded it. Viewers can’t necessarily tell from a video the information that officers had at the time, what the criminal background of the suspect may be nor whether the suspect resisted arrest, all of which factor into an officer’s use of force, he said.

Additionally, Buffington said, videos of police actions can be confusing or misleading. Buffington works with body cam footage regularly in his work. “Everyone’s moving so fast you get seasick watching these videos,” he said. Bystander footage is not necessarily of better quality. In legal cases against police officers, experts and attorneys comb through each frame of video evidence, watching for suspicious shadows to discern which way each gun may be pointing. A recent case handled by Buffington’s firm hinged on the gaze of a dog in a video’s background. “It’s really crucial that you have a trained eye when reviewing these videos, over and over and over at a frame by frame rate,” he said. “Each frame reveals clues.”

In the five months since Mobile Justice CA was released, Patrisse Cullors has used it twice. She said she used it once to film a stranger’s arrest in Santa Monica, and she used it once to film her brother when he had a “psychotic break.” Cullors grew up in Los Angeles, and when she was 16 her brother was arrested for evading an officer, she said. He was sent to LA County’s Twin Towers facility, where Cullors and her family allege that he was beaten by deputies. Cullors’ brother has since been diagnosed with schizoaffective disorder, she said. Recently, her neighbors called the police after he became disruptive. Cullors and her neighbors used Mobile Justice CA to document the arrest as the police took her brother away.

“It felt like I had an army behind me,” she said. “Oh yeah, the police saw me filming. I felt empowered.”

Correction: In a previous version of this article, we stated that Buffington handled a case that hinged on the gaze of a dog in the background. The case was managed by another lawyer in Buffington’s firm, not Buffington himself.