A photograph of Trump administration official Mike Waltz’s phone shows him using an unofficial version of Signal designed to archive messages during a cabinet meeting.

Mike Waltz, who was until Thursday U.S. National Security Advisor, has inadvertently revealed he is using an obscure and unofficial version of Signal that is designed to archive messages, raising questions about what classification of information officials are discussing on the app and how that data is being secured, 404 Media has found.

On Thursday Reuters published a photograph of Waltz checking his mobile phone during a cabinet meeting held by Donald Trump.

The screen appears to show messages from various top level government officials, including JD Vance, Tulsi Gabbard, and Marco Rubio.