Hackers broke into the US Fox News political Twitter account and posted messages saying President Barack Obama had been assassinated.
A series of six tweets coming from the FoxNewsPolitics account reported that Mr Obama had been shot to death in Iowa and the shooter was unknown.
In a statement posted on its website later, Fox News called the tweets "malicious" and "false." It said the hacking was being investigated.
Mr Obama plans to spend the July Fourth US Independence Day holiday at a barbecue at the White House with military families and administration staff.
The Secret Service would not comment on the tweets.
Fox's political Twitter account has more than 34,000 followers.