MICROSOFT SAID SOME of its most popular Web sites were knocked offline Thursday by an act of Internet vandalism known as a "denial of service attack." It claimed that the incident was separate from an internal technical problem that brought down its key Internet properties on Tuesday and Wednesday.

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The hack attack began Thursday morning and targeted the routers that direct traffic to the company's Web sites, Microsoft said in a statement issued Thursday afternoon. Access to some of the company's most popular online destinations, including the Microsoft.com home page and MSN.com Web portal, were intermittent for many customers throughout the morning, Microsoft said.

The software maker said it took steps to counter the attacks, and that its Web sites were operating normally by Thursday afternoon. The company also said it has been in touch with the U.S. Federal Bureau of Investigation (FBI) about the attacks.

DoS attacks disable a computer by bombarding it with a high volume of information requests in a short period of time, causing it to crash or become so overwhelmed that it grinds to a halt. A series of high-profile DoS attacks in February last year brought prominent Web sites, including Yahoo and eBay, temporarily to their knees, costing millions of dollars and raising new concerns about security on the Internet.

Thursday's incident comes just hours after Microsoft's Web sites were knocked offline for almost an entire day by what the company said was a technical error made by one of its engineers Tuesday evening. In that outage, a technician working on the company's DNS (domain name system) network changed the configuration on one of its routers, blocking access to its sites, Microsoft said Wednesday.

Microsoft said Thursday's incident was "completely separate" from the outage that started Tuesday. One security expert was skeptical, however, saying Microsoft's problems may have been the result of a DoS attack all along.

"It is hard to believe that one problem is different from the other," said Ric Steinberger, technical director at online security company SecurityPortal.

"It is not always obvious from the start that something is DoS or due to a change in configuration. I think probably when the dust settles, Microsoft is going to say this is all the result of a DoS attack."

DNS servers translate addresses, such as www.microsoft.com, into the numeric IP addresses that computers use to communicate with each other. When a DNS system fails, Web browsers can't locate the IP addresses assigned to Web servers and hence can't open the desired Web page.

"It is unfortunate that an individual or group of individuals would engage in this kind of illegal activity," Microsoft said in the statement.