A server problem at the US National Security Agency has knocked the secretive intelligence agency off the Internet.

The nsa.gov Web site was unresponsive at 7 a.m. Pacific time Thursday and continued to be unavailable throughout the morning for Internet users.

The problem was resolved at around 11 a.m. Pacific time, according to Web site measurement company Netcraft.

The Web site was unreachable because of a problem with the NSA's DNS (Domain Name System) servers, said Danny McPherson, chief research officer with Arbor Networks. DNS servers are used to translate things like the Web addresses typed into machine-readable Internet Protocol addresses that computers use to find each other on the Internet.

The agency's two authoritative DNS servers were unreachable Thursday morning, McPherson said.

Because this DNS information is sometimes cached by Internet service providers, the NSA would still be temporarily reachable by some users, but unless the problem is fixed, NSA servers will be knocked completely off-line. That means that e-mail sent to the agency will not be delivered, and in some cases, e-mail being sent by the NSA would not get through.

"We are aware of the situation and our techs are working on it," a NSA spokeswoman said at 9:45 a.m. PT. She declined to identify herself.

A similar DNS problem knocked Youtube.com off-line in early May.

There are three possible reasons the DNS server was knocked off-line, McPherson said. "It's either an internal routing problem of some sort on their side or they've messed up some firewall or ACL [access control list] policy," he said. "Or they've taken their servers off-line because something happened."

That "something else" could be a technical glitch or a hacking incident, McPherson said.

In fact, the NSA has made some basic security mistakes with its DNS servers, according to McPherson. The NSA should have hosted its two authoritative DNS servers on different machines, so that if a technical glitch knocked one of the servers off-line, the other would still be reachable. Compounding problems is the fact that the DNS servers are hosted on a machine that is also being used as a Web server for the NSA's National Computer Security Center.

"Say there was some Apache or Windows vulnerability and hackers controlled that server, they would now own the DNS server for nsa.gov," he said. "That really surprised me. I wouldn't think that these guys would do something like that."

The NSA is responsible for analysis of foreign communications, but it is also charged with helping protect the US government against cyber attacks, so the outage is an embarrassment for the agency.

"I am certain that someone's going to send an e-mail at some point that's not going to get through," McPherson said. "If it's related to national security and it's not getting through, then as a US citizen, that concerns me."

Anders Lotsson with contributed to this report.