The Associated Press news agency (AP) has said that hackers took control of its Twitter account and posted false tweets, including one saying two explosions rocked the White House and injured US President Barack Obama.
Tuesday's false tweet said there had been two explosions at the White House and that Obama was injured.
The attack on AP's Twitter account and the AP Mobile Twitter account was preceded by phishing attempts on AP's corporate network.
A group calling itself the Syrian Electronic Army on Tuesday claimed responsibility on its own Twitter feed for the AP hack.
FBI spokeswoman Jenny Shearer said the bureau was investigating how hackers were able to send out the bogus tweets.
The AP's Twitter account was suspended shortly after the fake tweet. The news organisation said the tweet came after hackers made repeated attempts to steal the passwords of AP journalists.
Paul Colford, AP spokesman, said the news cooperative has disabled its other Twitter accounts following the attack.
He added that the agency was working with Twitter to investigate the issue.
The false report briefly sent both the S&P 500 and Dow Jones Industrial Average sharply lower before they recovered.
White House spokesman Jay Carney told reporters soon after the tweet went out that Obama was fine.
"I can say that the president is fine," Carney said shortly after the fake tweet. "I was just with him."
It was the latest hacking incident involving news organisations. On Saturday, social media accounts maintained by CBS News programmes "60 Minutes" and "48 Hours" were compromised. The Syrian Electronic Army also claimed responsibility for some of these hacks on their own twitter account.