(Bloomberg) -- The Federal Emergency Management Agency on Sept. 20 will conduct the first test of a national wireless alert system that allows the president to send text messages directly to a majority of mobile-phone users in the U.S.

The message will be identified as a “Presidential Alert” and will contain the text: “THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed,” according to FEMA. Carriers across the country will participate in the test, meaning that most -- but not all -- mobile users will receive the message with no ability to opt out, the agency said in a statement last week.

FEMA developed the Wireless Emergency Alerts system under a 2016 law enacted by President Barack Obama. It specifies that the system can only be used to communicate about disasters or terrorist attacks. That means it’s unlikely that President Donald Trump, whose use of Twitter has revolutionized White House communications, could use it to send personal messages.

“The WEA system is used to warn the public about dangerous weather, missing children, and other critical situations,” the agency said.

Testing will start at 2:18 p.m. Eastern time and last for about half an hour, FEMA said. The agency said it will also conduct a separate test on Sept. 20 of a system that sends alerts via radio and television broadcasters. That system has been tested before.

To contact the reporter on this story: Chris Strohm in Washington at cstrohm1@bloomberg.net

To contact the editors responsible for this story: Craig Gordon at cgordon39@bloomberg.net, Ben Holland, Bernard Kohn

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