Computer hackers took over a Twitter account belonging to US broadcaster Fox News and sent messages falsely declaring that President Barack Obama was dead.
The messages began appearing on 4 July, the first reading: "Just regained full access to our Twitter and email. Happy 4th."
A stream of tweets followed claiming that Barack Obama had been assassinated in Iowa, the BBC reports.
The messages remained on the site for hours after they were posted.
A group or individual, calling themselves The Script Kiddies appeared to claim responsibility.
In a statement, Fox News said it would be requesting a detailed investigation from Twitter about the incident, along with measures to prevent further unauthorised access to its accounts.
The White House declined to comment. The Secret Service, which is charged with protecting the president, said it was looking into the incident, Reuters reports.
Mr Obama was celebrating the Independence holiday with his family at the White House and was due to host military families to watch Fourth of July fireworks in the evening.