The Web site and computer system at the Naval War College have been offline for more than two weeks following a mysterious hacking incident. The hacker broke into unclassified sections of the network, which include the e-mail system for staff and students, most of which are senior level members of the military.

While this is not the first time a Naval computer system has been taken offline due to an intrusion, one expert says two weeks is a long time for the system to be down.

"It's very unusual," said Richard Clarke, the former U.S. Cyber Security Czar and now an ABC News Consultant. "In addition to finding out how this happened, they may be doing a physical check on every computer to make sure there isn't a timebomb waiting for them, to ensure the data isn't ex-filtrated once they go back online."

A spokesperson at the Navy Cyber Defense Operations Command in Norfolk, Va., said they "detected an intrusion" in mid-November and that they decided to pull the plug and go offline in order to mitigate the problem and update the firewalls. Lt. Doug Gabos at NCDOC said the hacking is an isolated incident, and no other Department of Defense computers nor any classified information was compromised.

"The Washington Times" reported that Chinese hackers were responsible for the breech. Clarke, however, said that in many cases hackers will use a Chinese router as their 'last hop' in order to make China look like the origin attack.

Some previous incidents have been traced back to Chinese hackers, but no previous incidents have been traced back to the Chinese government.

While this is the first attack on the system at the Naval War College, there have been a series of attacks on DOD computer systems in the past few years, which U.S. authorities have codenamed "Titan Rain."

"Typically they originate overseas," said Clarke. "They defeat our cyber-defenses and firewalls which show they are quite sophisticated."

In most incidents, once inside the hackers or viruses begin copying databases. After that, "we don't know where they go," said Clarke.

Before Titan Rain, there was Moonlight Maze in the late 1990s, in which U.S. authorities discovered intrusions into the Pentagon and other DOD computer systems. Investigators traced the hacking back to some routers in Moscow, but it is still unknown from whom and from where those hackings originated. The case is still being investigated.