False tsunami warning messages flashed up on mobile phones along the United States East and Gulf Coasts when a US National Weather Service (NWS) systems test went badly wrong.
The false alerts on Tuesday morning appeared to have been sent out by the private forecasting company AccuWeather, according to images posted on social media.
AccuWeather, however, pointed the finger at the NWS.
The NWS and the Federal Communications Commission said they were investigating.
It was not the first time this year Americans were roused by their cellphones warning of an impending catastrophe. Last month's false alarm was a missile headed for Hawaii.
The National Weather Service said its National Tsunami Warning Center had issued a routine monthly test message that had been misconstrued, spooking people in cities as far apart as Boston and Houston.
"The test message was released by at least one private sector company as an official Tsunami Warning, resulting in widespread reports of tsunami warnings received via phones and other media across the East Coast, Gulf of Mexico, and the Caribbean," the service said in a statement.
At least some people who clicked the alert to read the full message saw a disclaimer saying the alert was in fact a test, according to screenshots posted online.
AccuWeather said the National Weather Service had wrongly coded the test as a real warning, confusing its automated alerts system.
"AccuWeather has the most sophisticated system for passing on NWS tsunami warnings based on a complete computer scan of the codes used by the NWS," the company said in a statement.
"While the words 'TEST' were in the header, the actual codes read by computers used coding for real warning, indicating it was a real warning."
AccuWeather said it had warned the National Weather Service in 2014 of this vulnerability after a similar error.
Last month, the state of Hawaii mistakenly warned residents of an inbound missile, sparking panic. The message was later blamed on an error by an employee, who was subsequently fired.
