Former National Security Agency contractor Edward Snowden sent an ominous warning Wednesday after hundreds of millions of electronic devices all across the U.S. received a test of a “presidential alert."

In a tweet, Snowden said Americans should take heed of the rapid advance of technology and ask themselves a simple question.

"All our lives dangle at the end of a wire. Ask yourself: who controls it? How else might it be used? #PresidentialAlert," he said.

Smartphones began buzzing at 2:18 p.m. ET when a nationwide wireless emergency test, managed by Federal Emergency Management Agency, was sent out.

"THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed," said the message, under a banner that said "Presidential Alert."

TVs and radio got a similar message soon after.

FEMA is required to run a cellphone notification test, used to warn of a myriad of public safety issues, every three years in accordance to a law passed in 2016.

Despite it being dubbed a "Presidential Alert," President Trump, nor any president, can send out any message they please.

[Related: Three New Yorkers suing to stop test of presidential alert system on Wednesday]

"Except to the extent necessary for testing the public alert and warning system, the public alert and warning system shall not be used to transmit a message that does not relate to a natural disaster, act of terrorism, or other man-made disaster or threat to public safety," says the Integrated Public Alert and Warning System Modernization Act of 2015 signed by former President Barack Obama.

As a layer of security to prevent abuse, messages must be approved by multiple FEMA officials.

Snowden, who was granted asylum in Russia back in 2013 after he leaked secret information from the National Security Agency's surveillance programs and has been there ever since, has routinely chimed in on Twitter with warnings about the government and technology.

For instance, in March, Snowden dubbed Facebook a surveillance company rebranded as "social media" after it was discovered the social media giant suspended Cambridge Analytica, a data analytics firm which worked worked for President Trump’s campaign, accusing the firm on of not deleting data it had improperly harvested from Facebook users, which number in the tens of millions.