The Associated Press has said its Twitter account has been hacked, after the posting of a bogus post about explosions at the White House.
The news agency's account was suspended and it advised all tweets should be ignored until further notice.
The false message said: "Breaking: Two Explosions in the White House and Barack Obama is injured."
US markets were spooked by the tweet; the Dow Jones Industrial Average dropped 150 points as it was retweeted.
On Tuesday evening, the FBI said it was investigating the incident.
White House spokesman Jay Carney told reporters soon after the tweet was published that President Obama was fine, and financial markets quickly recovered the lost ground.
AP said later: "The @AP twitter account has been hacked. The tweet about an attack at the White House is false."
The account was back online on Wednesday.
The agency said the attack came after repeated attempts by hackers to steal the passwords of AP journalists, adding that it was working to correct the issue.
Spokesman Paul Colford said the wire service had disabled other Twitter accounts following the attack and was working with the microblogging site to investigate the breach.
AP reporter Mike Baker tweeted: "The @AP hack came less than an hour after some of us received an impressively disguised phishing email."
The phishing email in question appeared to come from the email account of another AP staffer, with a request to click a link to a "very important" article on the Washington Post website.
The link opened a bogus login page requesting users enter their staff ID and password, reports suggest.
A group claiming to support Syrian President Bashar al-Assad took responsibility for the attacks, tweeting: "Ops! @AP get owned by Syrian Electronic Army! #SEA #Syria #ByeByeObama."
The Syrian Electronic Army claims to have been behind similar attacks on the twitter feeds of Agence France Presse news agency, Sky News Arabia, al-Jazeera mobile and CBS News.
Two of the US broadcaster's accounts were suspended after fake messages appearing on the @60Minutes account criticised US support for "terrorist" rebels in Syria.
On Tuesday evening, Twitter said it was investigating the incident and reminded users to be "extra vigilant" about phishing emails.