The CIA is conducting a cyber-war game this week geared to simulate a major Internet attack by enemy computer hackers, an intelligence official said Thursday.

Dubbed "Silent Horizon," the three-day unclassified exercise is based on a scenario set five years in the future and involves participants from government and the private sector.

"These are people who could likely be affected or enlisted in a real situation," the intelligence official said.

"Its goal is to help the United States recognize indicators of a large-scale cyber attack."

The exercise was being conducted in Charlottesville, Virginia, by members of the CIA's Information Operations Center, which evaluates foreign threats to U.S. computer systems, particularly those that support critical infrastructures. It was expected to conclude Thursday.

The federal government has conducted various attack simulations since the Sept. 11, 2001, attacks on New York and Washington, which killed about 3,000 people and prompted the U.S. war on terrorism.

Top U.S. intelligence officials say it may be only a matter of time before the United States is attacked again by terrorist groups including
Osama bin Laden's al Qaeda.

Cyber attacks, which have drawn less publicity than possible chemical, biological, radiological and nuclear attacks, are viewed by U.S. officials as a potential al Qaeda weapon against the U.S. economy.

Online crime has exploded in recent years, a result of organized crime groups based in Eastern Europe. But investigators so far have uncovered few links to Islamic extremists.

"We have not uncovered any significant links to terrorism," said Brian Nagel, assistant director of investigations for the U.S. Secret Service, in an interview with Reuters last week.

But there are some signs that Islamic extremists are getting into the act.

An Indonesian man named Imam Samudra, who was found guilty of the 2002 Bali nightclub blasts, included a chapter entitled "Hacking: Why Not?" in his autobiography.

While hackers have uncovered holes in power plants and other infrastructure, experts say terrorists are likely to favor conventional attacks as long as they are possible.

"When it's really too hard to bring kinetic weapons in ... the bad guys will turn to cyber attacks," said Allan Paller, chief executive of the SANS Institute, a nonprofit security-training organization.