Hackers took control of the Associated Press Twitter account overnight and sent a false tweet about two explosions at the White House that briefly sent US financial markets reeling.
In the latest high-profile hacking incident involving social media service Twitter, an official @AP account reported that two explosions at the White House injured president Barack Obama.
AP spokesman Paul Colford quickly confirmed the tweet was "bogus," but within three minutes of the tweet hitting the web virtually all US markets took a plunge on the false news, in what one trader described as "pure chaos."
White House spokesman Jay Carney told reporters that Mr Obama was fine soon after the tweet went out a little after 1:00pm local time.
Markets quickly recovered their losses after the tweet was knocked down.
Some traders blamed automatic electronic trading for the sharp fall and bounce back.
At a time when cybersecurity and hacking have become top national security concerns, Twitter and its reach to hundreds of millions of users is coming under growing scrutiny for the risk of data breaches on the site.
A group calling itself the Syrian Electronic Army, which is supportive of that country's leader Bashar al-Assad, claimed responsibility on its own Twitter feed for the AP hack.
The group, which creates new Twitter accounts every time the company suspends its old ones, has recently also claimed credit for similar hacks of Twitter accounts for National Public Radio, BBC and CBS's 60 Minutes program, among others.
The AP's two Twitter accounts, @AP and @AP_Mobile, were suspended shortly after the fake tweet, and the news organisation later reported hackers had previously made repeated attempts to steal the passwords of AP journalists.
A Twitter spokesman declined to comment on the AP breach, saying the company does not comment "on individual accounts for privacy and security reasons."
An FBI representative had no immediate comment on the AP hacking incident.
The spate of recent hacks have again turned the spotlight on what critics deem to be Twitter's thin security offerings.
For years, security experts have called on the San Francisco company to introduce a measure called two-factor authentication that they say would greatly reduce such breaches.
In recent months, Twitter has appeared to respond by hiring security experts and posting job listings for software engineers who could help the company roll out two-factor authentication.