Lulz Security has claimed responsibility for recent attacks on the Senate, Sony Corp, News Corp and the US Public Broadcasting System television network.
The CIA site initially could not be accessed from New York to San Francisco, and Bangalore to London. Later in the evening service was sporadic.
"We are looking into these reports," a CIA spokesman said.
Lulz Security has defaced websites, posted personal information about customers and site administrators, and disclosed the network configurations of some sites.
Security analysts have downplayed the significance of these attacks, saying the hackers are just looking to show off and get as much attention as possible.
In the case of the CIA attack, hackers would not be able to access sensitive data by breaking into the agency's public website, said Jeffrey Carr, author of the book Inside Cyber Warfare: Mapping the Cyber Underworld.
Lulz only made claims that it attacked www.cia.gov, and there was no evidence that sensitive data in the agency's internal computer network had been compromised.
There also were no apparent links to more serious network security breaches recently at the
International Monetary Fund and Lockheed Martin Corp. Lulz Security has not been linked to those incidents.
Lulz, whose members are strewn across the globe, announced the attack shortly before 6pm East Coast time.
"Tango down," the group Tweeted, pointing to www.cia.gov.
Although the group, also known as Lulz Boat, fashions itself more as pranksters and activists than people with sinister intent, its members have been accused of breaking the law and are wanted by the FBI and other law enforcement agencies.
Lulz broke into a public website of the US Senate over the weekend and released data stolen from the legislative body's computer servers.
In May, the group posted a fake story on the PBS website saying that rapper Tupac Shakur was still alive and living in New Zealand. Shakur was murdered in 1996.
more at Telegraph
