Hackers spooked markets Tuesday after breaking into the Associated Press's Twitter account and falsely reporting President Barack Obama had been injured after two blasts at the White House.
A brief alert on the news agency's @AP account read: "Breaking: Two explosions in the White House and Barack Obama is injured."
Almost immediately the wire service posted via its corporate communications feed that its @AP Twitter account had been hacked, before promptly suspending the service.
"Advisory: @AP Twitter account has been hacked. Tweet about an attack at the White House is false. We will advise more as soon as possible," @AP_CorpComm posted.
AP spokesman Paul Colford later said the wire service had disabled other Twitter accounts following the attack and was working with the micro-blogging site to investigate the breach.
The FBI also said it was investigating the incident.
White House spokesman Jay Carney told reporters the president was unharmed. "I can say that the president is fine," Carney said shortly after the fake tweet. "I was just with him."
Stock markets plunged just as the report came out, with the Dow Jones Industrial Average losing 130 points, or 0.9 percent, and the S&P 500 dropping 12 points, or 0.8 percent.
Just as quickly they rebounded to where they were before the tweet, all within three or four minutes.
Online activists backing the regime of Syrian President Bashar al-Assad later claimed responsibility for the hack -- the latest in a series they have orchestrated against high-profile news organizations.
The group -- the so-called Syrian Electronic Army -- also successfully hacked the Twitter feeds of AFP and CBS News earlier this year.
"Ops! @AP get owned by Syrian Electronic Army! #SEA #Syria #ByeByeObama," the group posted after Tuesday's hack.
The group has also previously claimed credit for hacking the websites of Sky News Arabia and Al-Jazeera Mobile.
On Saturday, social media accounts connected to CBS News programs "60 Minutes" and "48 Hours" were also targeted.
A tweet on the @60Minutes Twitter account stated: "Exclusive: Terror is striking the #USA and #Obama is shamelessly in bed with Al-Qaeda."
The post was later deleted, with CBS News saying: "We have experienced problems on Twitter accounts of @60Minutes & @48Hours; We apologize for the inconvenience; Twitter is resolving issues."
The Syrian Electronic Army had also claimed responsibility for hacking the Twitter account of world football supremo Sepp Blatter.
Blatter, the president of football's global governing body FIFA, was targeted in a series of bogus tweets which announced he was stepping down because of corruption charges.