The FBI is investigating how hackers managed to send a fake Associated Press tweet announcing an attack on the White House.
FBI spokeswoman Jenny Shearer confirmed an investigation, but did not provide further details, Reuters reported.
The US Securities and Exchange Commission is also reported to be investigating.
The Dow Jones took a dive on Tuesday moments after the tweet announced that the White House had been attacked and Barack Obama was injured.
Just after 1:00pm (eastern time) , AP tweeted:
The news quickly spread with many questioning the veracity of the tweet and whether or not the news organisation's account had been hacked.
A message from AP's corporate communications account soon confirmed the hack:
However, the news of the fake didn't come soon enough for Wall Street traders with the Dow Jones briefly tumbling:
The White House quickly confirmed that the president was fine, as CBS reported:
AP is among a number of high profile twitter accounts that have been hacked recently, including a number for CBS News including the 60 Minutes, 48 Minutes and CBS Denver feeds. The Syrian Electronic Army was reportedly responsible.