What is Signal, the chat app used by US officials to share attack plans?

A magazine journalist’s account of being added to a group chat of US national security officials coordinating plans for airstrikes has raised questions about how highly sensitive information is supposed to be handled.

Atlantic editor-in-chief Jeffrey Goldberg detailed a discussion that happened over the Signal messaging app hours before strikes on Iran-backed Houthi-rebels in Yemen ordered by US President Donald Trump.

The National Security Council has since said the text chain “appears to be authentic” and that it is looking into how a journalist’s number was added to the chain.