The Lulz Security hacker group claimed to have briefly knocked the public website for the U.S. Central Intelligence Agency offline on Wednesday.
"We are looking into these reports," a CIA spokeswoman told Reuters.
LulzSec hackers have claimed in recent weeks to have cracked into Sony, Nintendo, the US Senate, the Public Broadcasting System news organization and an Infragard company that works with the FBI.
"It's becoming a big problem, because at the end of the day these guys are doing whatever they want," said Panda computer security labs technical director Luis Corrons. "This is showing us that we have a long way to go to protect our systems and our information."