The US Secret Service has launched an investigation into alarming fake messages that president Barack Obama was dead posted by hackers on the Fox News Twitter account.
The hackers wrote six tweets, reporting that Mr Obama had been shot to death in Iowa.
The @foxnewspolitics feed stated: "BREAKING NEWS: @BarackObama assassinated, 2 gunshot wounds have proved too much."
The messages remained online for more than two hours before they were removed, Reuters has reported.
Fox News said the tweets were "malicious" and "false" and it will also conduct an investigation.
"We will be requesting a detailed investigation from Twitter about how this occurred, and measures to prevent future unauthorised access into FoxNews.com accounts," Fox News Digital vice-president and general manager Jeff Misenti said.
"Those reports are incorrect, of course, and the president is spending the July 4 holiday with his family," Fox News said.
The White House declined to comment but the Secret Service, which is charged with protecting the president, said it was looking into the incident.
"The Secret Service is investigating the matter and will conduct the appropriate follow-up," spokesman George Ogilvie said.
The first hacked tweet appeared about 2am (local time) and said: "@BarackObama has just passed. The President is dead. A sad 4th of July, indeed. President Barack Obama is dead."
The next one, "@BarackObama has just passed. Nearly 45 minutes ago, he was shot twice in the lower pelvic area and in the neck; shooter unknown. Bled out."
The six messages were removed about noon on Monday, about 10 hours after being posted, but not before attracting a flurry of attention.
Because of the seriousness of the content, senior Secret Service officials held a conference call Monday morning to discuss the posts, said a law enforcement official who requested anonymity because of the investigation into the matter.