Washington (CNN) The "presidential alert" headed to Americans' cell phones won't actually be written by President Donald Trump, the Federal Emergency Management Agency said Tuesday.

Wednesday afternoon's message is the first nationwide test of the system built by the government and cell phone carriers to push an emergency message to nearly all cell phones in the US, a senior FEMA official told reporters on a Tuesday conference call.

Despite the name, "the president will not originate this alert, say, from his mobile device," the official said. "You would not have a situation where any sitting president would wake up one morning and attempt to send a particular message."

That name had generated questions about how and when the system can be used. The official said the uses are limited, such as to a "coordinated attack on our major cities across the country" or "some other type of public peril that is ongoing in the country at the time."

The FEMA official spoke to reporters on a conference call the agency arranged on the condition of anonymity. FEMA did not elaborate on why the official could not be named.

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