In the early morning of September 9, 2016, Bill Moore, CEO of the Austin-based walkie-talkie app company Zello, contacted the Middle East Media Research Institute. He was seeking a copy of a report MEMRI had recently published describing how ISIS members and supporters were using Zello, which allows people to send voice messages to each other in private and also public channels. Moore had learned about the findings through a Google Alert.

“Can you share a copy of the report explaining ISIS uses Zello? I'm the CEO of Zello,” the message read, according to emails reviewed by WIRED and confirmed by Moore and MEMRI.

Hours later, MEMRI deputy director Elliot Zweig sent him the report. While MEMRI hadn't collected actual messages, its findings included screenshots of Zello users whose avatars featured photos of ISIS’s iconic black flags, and public channels with names like “Islamic State Channel.” One channel called simply “Jihad.” described itself this way: “For the Brothers who desire to be with Mujahideen & to talk about Jihad and Islam.” Some of the channels had been advertised by ISIS sympathizers on another encrypted app, Telegram. The list wasn't comprehensive, and didn't reference any specifically troubling conversations; it was just a snapshot of accounts and channels MEMRI found easily at the time.

In his response, Zweig offered to connect Moore with the executive director of MEMRI, Steven Stalinsky. When he heard nothing back, Zweig extended the offer again. This time, Moore replied: “Confirming receipt, thank you very much. No need from our side to discuss now.”

A few weeks later, Zweig tried one more time. “We note that the ISIS and other jihadi accounts mentioned in our report are still active on your service, and would like to again offer the opportunity for a briefing/conversation with our executive director,” he wrote.

Moore never got back in touch—that is, until just last week, when MEMRI published yet another report showing that nearly all of the channels it flagged in 2016 were still live.

'We need to do a better job.' Zello CEO Bill Moore

Over those 18 months, Zello's audience has grown dramatically. The app, which launched in 2011 and now boasts 124 million users worldwide, briefly topped the US App Store last fall when hurricane victims in Texas and Florida used it to communicate with rescuers. In a single week last September, Zello accrued six million new users. The company has also scaled its paid, enterprise product, called ZelloWork, which is used by major hotel chains and retailers.

Unlike other chat apps, including WhatsApp and Telegram, Zello is almost entirely voice-driven. Users can join public channels about a given topic to hear what other members are saying, sort of like a two-way radio. Or they can create private, encrypted channels with individuals or groups. Zello doesn't retain a record of any public voice messages. But those voice messages do get saved onto the phones of other users who have heard them.

Over the years, Zello has also found itself, like so many tech platforms, associated with terrorist activity. Last April, a man who killed five people and injured 14 others in Stockholm by driving a truck into a crowd reportedly used Zello to discuss his plot before, during, and after the attack. Meanwhile, Moore says the company has received subpoenas from law enforcement seeking to monitor terror suspects on Zello. And then there are the MEMRI reports.

Despite these signals, the Austin-based company has apparently taken a largely passive stance toward policing terrorist-affiliated public channels and accounts. Even as larger tech companies like Facebook, YouTube, and Twitter have faced years of congressional interrogation and public pressure over how they filter similar content, Zello has dedicated limited resources to this type of moderation. The company relies entirely on users to flag problems, an action that doesn't always guarantee results.