Last month, video footage emerged that appeared to show something illegal: A U.S. marshal approached a woman who was filming him on duty, snatched her smartphone, and smashed it on the ground.

That incident only became news because someone else was filming the encounter. But not every bystander filming a police encounter can have a backup. What should a person do when there's no one else on the scene?

A new app tries to answer this question by offering, in effect, a different kind of backup. Called Mobile Justice CA, the app uploads all video footage as it’s being captured to servers owned by the American Civil Liberties Union (ACLU). Even if the phone is destroyed, the video will survive.

The app was co-released Friday by the ACLU of Southern California and the Oakland-based Ella Baker Center for Human Rights, and it’s available now for iOS and Android devices.

Mobile Justice CA does more than automatically upload video. It includes a “witness” button, which a user can press to notify other app users within a three-mile radius that they are observing a police interaction. It also lets users file written reports with a local ACLU office and includes versions of the ACLU’s “Know Your Rights” guides for photographers, protesters, and citizens.

California isn’t the first state to get an app like this. Missouri’s ACLU chapter released similar software during protests in Ferguson last year, and a New York-specific app focused on stop-and-frisk has been out since 2012.

The ACLU and the Ella Baker Center have found fast success with their version, though: Since its release on Friday, Mobile Justice CA has been downloaded almost 40,000 times. (New York’s “Stop and Frisk Watch” has been downloaded about 30,000 since its release in 2012.)

Patrisse Cullors, director of the truth and reinvestment campaign at the Ella Baker Center and one of three co-founders of the #BlackLivesMatter movement, told me that the app is only one of several forthcoming measures meant to serve as a check on law enforcement.

Within a year, Cullors told me, the Center plans to debut a “web-based platform” that will help communities track behaviors—both positive and negative—among law-enforcement agencies and individual police officers. Cullors described the platform as “a Facebook for challenging criminalization in your community.”

The platform is intended to connect to Mobile Justice CA’s reporting feature, and to help organizers compare trends across departments. It will also allow local community members to track their own law enforcement agencies and to note which officers seem to regularly abuse residents, enabling the Center to compare patterns of abuse between departments.

“Oftentimes, communities know their local law enforcement really well,” said Cullors.

For now, there’s the app. Video uploaded to ACLU servers will be reviewed by the organization’s lawyers, but it will still belong to the person who captured it.

Body cameras have been hailed as a solution to police brutality, and in that they’ve proven popular but fraught: They improve officer accountability while functioning as one more surveillance tool in communities often already riddled with them. Most of the videos that have ignited recent protests and reforms around the country, meanwhile, have been shot by bystanders, not officers themselves.

It’s always easy to spend more money on police departments, even in the name of oversight. Mobile Justice CA is something different. It builds a more stable technological infrastructure for bystander-provided video—a potentially less intrusive form of surveillance when used on citizens, and a seemingly more powerful one when used on cops.