Hackers have targeted the main Twitter account of The Associated Press news agency, sending out an erroneous tweet about an attack at the White House.
The tweet, which said that there had been two explosions at the White House and President Barack Obama was injured, came after hackers made repeated attempts to steal the passwords of AP journalists.
The AP said that its Twitter account had been suspended following a hack and said it was working to correct the issue.
The tweet put out by hackers briefly sent the Dow Jones Industrial Average sharply lower. It fell about 143 points, from 14,697 to 14,554, after the fake Twitter posting, but quickly recovered.
A Securities and Exchange Commission spokeswoman declined to comment on the incident.
AP spokesman Paul Colford said the news co-operative is working with Twitter to investigate the issue. The AP has disabled its other Twitter accounts following the attack, Mr Colford added.
White House spokesman Jay Carney said the president is fine. "I was just with him," Mr Carney said at a news briefing.
A representative for Twitter did not immediately return messages for comment.