More than 200 million US mobile phones have received a test “Presidential Alert” notification.
The trial is designed to check that a previously unused emergency communications system works properly.
Unlike other alerts, such as natural disaster warnings, there was no way to opt out, except switching a device off or otherwise blocking its connection.
Some have described the test as a “Trump Alert”, but the US leader was not personally involved in the trial.
Instead, the nationwide event is being run by the Federal Emergency Management Agency (Fema), which would also be in direct control of the system if a real alert was ordered by the US President, Donald Trump.
It is intended to be used to warn of major threats, including, missile attacks, acts of terrorism and natural disasters. The test was mandated under a 2015 law that said one must be run at least once every three years.
It was originally scheduled for September but was delayed in order to avoid any confusion while Hurricane Florence response efforts were continuing in North and South Carolina.
After the alert, Twitter and Facebook immediately lit up with people discussing the merits of the system or complaining that they had not received the notification.