The US National Security Agency’s Web site was offline for several hours on Friday, fuelling speculation that hackers had disabled it.
The site first went down at about 2 pm (1800 GMT), according to a Web site that tracks the downtime of other sites.
“Nsa.gov is DOWN for everyone. It is not just you. The server is not responding...” a message posted on isitdownrightnow.com said.
The NSA site was still down as of 9 pm (0100 GMT Saturday).
Some Twitter accounts claiming to be associated with the hacker group Anonymous took responsibility, according to NBC News. But it pointed out that those claims only appeared after the site had been down for several hours.
The nsa.gov site is primarily informational for the public.
The NSA said an internal error that occurred during a scheduled update was the reason the Web site was inaccessible.
“The issue will be resolved this evening,” a spokesman said, and rejected claims that the outage was caused by a distributed denial of service attack.
The agency has been at the centre of controversy since fugitive whistleblower Edward Snowden began releasing information about its surveillance capabilities and practices.
The most recent revelations said the intelligence service monitored phone calls from 35 international leaders, including German Chancellor Angela Merkel.