Hackers direct their anger online.(Chicago Tribune)

Claiming they are patriots and not vandals, hackers have begun attacking Web sites perceived -- or misperceived -- to be somehow linked to terrorism, prompting federal authorities to urge computer systems administrators to step up security.

Hackers largely have focused their attacks on the Middle East, forcing Web sites in Afghanistan to shut down and defacing the Iranian Ministry of the Interior's Web site by posting a defaced image of Osama bin Laden.

But hackers also tried to bring down the Web site run by the American Civil Liberties Union of Illinois through a "denial of service" attack that floods the page with so much traffic that the site shuts down. It is unclear why the hackers chose that site, but the ACLU speculates that their record of defending civil liberties could be construed as an impediment to fighting terrorism.

"We began noticing the activity after (Sept. 11) and it seems to be ongoing," said Ed Yohnka, spokesman for the ACLU of Illinois.

And hackers took aim--perhaps inadvertently--at two Web sites owned by the Chicago-based insurance firm Aon Corp., which has been unable to account for about 200 of its 1,350 workers based in New York's World Trade Center.

The Web sites were operated by a unit of Aon called the Special Risks Terrorism Team and had the word "terrorism" in their Internet addresses. A hacker who identified himself as "RaFa" replaced the Web sites' home pages and posted a message declaring: "Those who choose to harm our families, our friends and our countrymen have and will continue to become our aim. . . . We have already disabled a number of Palestinian (Internet service providers), and now are aiming at those ISPs and Web servers based in Afghanistan with the intent to destroy them."

Aon spokesman Stephen Ban declined to comment.