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:
acts of terrorism
The alert produced a tone and showed a notification saying: “THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed.”
It was being broadcast from mobile phone masts for about 30 minutes from 14:18 ET (19:18 BST) – with devices displaying only a single message.
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.
Others posted parody alerts, featuring photoshopped images.