The US Federal Bureau of Investigation (FBI) will investigate this week's attack of a half-dozen prominent Web sites, US Attorney General Janet Reno said at a press conference today.

Yahoo, Amazon.com, eBay, and Buy.com were targeted with denial of service attacks earlier this week, and online brokerage ETrade Group and ZD's technology news site ZDNet confirmed they had also been attacked.

G-men on the case The FBI is in contact with the affected companies as part of its assessment of the situation, according to Debbie Weierman, an FBI spokeswoman.

Weierman said: "Any type of transmission of a program information code or command to a Web site resulting in intentional damage is a federal offence. The FBI has jurisdictional authority to investigate these crimes."

The targeted companies who could be reached for comment today, including Amazon.com, eBay and eTrade, said they had corrected any problems. They would not comment further, apparently afraid to give hostile parties any more ammunition.

Amazon woman Patty Smith, a spokeswoman for Amazon, said: "The minute you start to talk about security is the minute it starts to be degraded."

"Denial of service attacks have been around for a long time," said Jamie Lewis, chief executive officer of Burton Group, a network planning services company in Utah.

A denial of service attack means overloading a Web site. In these attacks, routers connecting the sites to the rest of the Internet have been flooded. There’s so much malicious traffic that the router becomes unable to cope. Once this is achieved and the site is overloaded, genuine users find themselves unable to get connections.