The US spy agency's cia.gov website stopped responding and members of a hacker group called Lulz Security claimed credit in a message on Twitter at @LulzSec.
"Tango down - cia.gov," the message read. "For the lulz."
Attempts by AFP to reach cia.gov were met with a message saying the Web page was not available.
Cia.gov was back online within two hours.
Asked about the Lulz Security claim, a CIA spokesman said "We are looking into these reports."
Lulz Security has claimed in recent weeks to have staged attacks on the online operations of Sony, Nintendo, the US Senate, the Public Broadcasting System news organisation and Infragard, a company that works with the FBI.
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