Since the COVID-19 pandemic first came to Los Angeles in the spring, the county Department of Public Health has hired nearly 2,600 people to do the manual work of contact tracing: asking people who test positive for the coronavirus to list everywhere they’ve been and everyone they’ve seen in recent days, then tracking down anyone they’ve encountered and testing them before they spread the virus further.

But the rate of community spread in Los Angeles is overwhelming the county’s capacity, said L.A. County Public Health Director Barbara Ferrer. That’s why she and other civic leaders gathered Wednesday night in a ballroom atop City Hall to encourage Angelenos to download an app called SafePass, developed by the tech company behind the popular but controversial safety-alert app Citizen.

Here’s how SafePass works: Users download the app and give it permission to use their phone’s short-range Bluetooth signal. Then, whenever the app detects another SafePass user nearby for more than five minutes, it logs that user as a contact. A user who tests positive for the coronavirus can upload their test result into the app, which automatically sends out an alert to everyone logged as a contact: You’ve been exposed to the virus, please get tested and isolate yourself if necessary.

That alert is anonymous—it says only that a user has been exposed, not who did the exposing—and the company behind the app, Sp0n Inc., says it automatically deletes users’ data after 30 days to preserve their privacy (and within 24 hours if a user deletes the app).

Google and Apple, which between them control the operating systems of nearly all the phones in the U.S., announced plans to create a similar Bluetooth-based contact tracing system in April. At the time, critics said the system was a poor replacement for labor-intensive manual contact tracing because it is effective only when a critical mass of users all have the app turned on. An Oxford University study found that 60% of a population needs to opt in for a Bluetooth-based app to stop the spread of the coronavirus.

But with the county’s army of manual contact tracers working in the background, L.A. leaders are betting that any additional contact tracing helps—even if that help comes from a tech company whose approach to public safety has often come under fire.

Citizen first launched in 2016 as Vigilante, an app that alerted users to nearby 911 calls and encouraged them to rush to the scene to share live video of what was going down. Critics quickly highlighted the risk of encouraging racial profiling, harassment and dangerous behavior, and within two days, Apple pulled Vigilante from its App Store over safety concerns.

The following spring, the app relaunched as Citizen, with new messaging that focused more on user safety than anti-crime vigilantism and, a few months later, a new round of $12 million in venture capital funding. The new version removed prompts urging users to get footage of active crimes but still allowed users to submit their own incident reports. In the years since, the company says, its user base has grown to more than 5 million, even as critics continue to argue that the app fosters racist surveillance and presents users with warped visions of crime-ridden cities when crime rates are at historic lows.

Citizen expanded to Los Angeles in spring 2019. In just the last 18 months, the company says, it has assembled a user base of nearly 1 million in L.A. County.

“Our mission is to keep our users safe,” said Andrew Frame, Sp0n’s chief executive. “So when the pandemic hit, there was a lot of discussion within the company…. ‘It seems like COVID is the No. 1 safety issue right now. What can we do?'”

The company first rolled out an experimental contact tracing feature on the Citizen app itself in the spring, asking users if they’d like to opt into the feature and give the app access to their Bluetooth antennas.

“An enormous amount of our user base immediately opted in,” Frame said, including more than 100,000 users in Los Angeles.

An Apple rule concerning contact tracing apps forced the company to spin SafePass off into a standalone app.

Frame said despite Citizen’s use of police and fire scanner feeds in its app, the company has no official relationship with law enforcement and will not share users’ data without their consent unless authorities have a search warrant or a subpoena. “There is no data sharing or deal, cooperation, or anything with law enforcement,” Frame said. “We’re doing this as a community good.”

L.A. leaders saw Citizen’s large user base, the company’s proven technical ability to handle millions of users and the fact that the app came at no cost to users or taxpayers as selling points that were difficult to resist.

“Contact tracing saves lives, and our city and county leadership have deployed every possible tool to slow the spread of this virus,” Los Angeles Mayor Eric Garcetti said in a statement. “Given Citizen’s immense capacity, millions of existing users, and focus on data privacy, we knew we could hit the ground running with SafePass.”

L.A. isn’t the first city to partner with Citizen for digital contact tracing. Stockton Mayor Michael Tubbs, who has made headlines for his ambitious experiments with universal basic income, began urging Stocktonians to download SafePass in August.

Tubbs said he had some initial concerns over data privacy, but after the company laid out its safeguards, he was sold.

“I think at this juncture, in the city and as a state, we need every tool in the arsenal,” Tubbs said. “The more people on the app, the more effective the app is, but if it helps alert one, 10, a thousand, or any number of people greater than zero” that they’ve been exposed to the virus, that’s a success.

SafePass users are able to get a free coronavirus test delivered to their home through a partnership with the genetic healthcare company Phosphorus. Frame said his company is losing money on the service as a whole and has no plans to monetize SafePass, but he said the expense is worth it to advance the company’s mission and get the world back to a point where the coronavirus is contained and the lockdowns can end.

The free publicity that comes from being the official digital contact tracer of a county of 10 million can’t hurt either. SafePass users don’t need to download the Citizen app, but the same login will work for both.