"WikiLeaks supporters, LulzSec, take down CIA ... who has task force into WikiLeaks," the WikiLeaks post on the Twitter social-networking and microblogging service said late Wednesday.
The post was monitored by United Press International.
"That we did. And that we do. <3," a follow-up post identified as coming from the hacker group Lulz Security said. "<3" is online shorthand for a heart.
A later post attributed to LulzSec said: "Goodnight twitter. The CIA anti-lizards will probably rise from the packet sea while we rest our shining-yet-saturated power field." The hacker group claims to be leading a "lizard rebellion."
The CIA said its Web site had malfunctioned Wednesday but would say nothing more about the incident, other than, "We're looking into the matter," The Wall Street Journal reported.
The site was back up early Thursday, a UPI review indicated.
There was no evidence sensitive data in the agency's internal computer network had been compromised, International Business Times reported.
WikiLeaks became the focus of a global debate over its role in the release of thousands of confidential messages about the wars in Iraq and Afghanistan and the conduct of U.S. diplomacy around the world.
The once-fringe, non-profit organization, which seeks to expose secret information about governments and corporations, was founded in 2006 by Australian activist-journalist Julian Assange, along with a group of like-minded activists and computer experts.
The purported attack on the CIA's public information site was the latest in a spree of high-profile attacks by LulzSec, including on an FBI affiliate site and the site of the Public Broadcasting Service.
LulzSec also claimed to have attacked Japanese technology giant Sony Corp. and some videogame companies.
It posted information this week from the U.S. Senate's public Web site, along with a comment, "Is this an act of war, gentlemen?"
LulzSec draws its name from the neologism "Lulz" -- from the Internet slang "LOLs," or "laughing out loud" -- and "Sec," short for "Security."