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
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