RALEIGH, N.C. — A company that provides online advertising services for WRAL.com and many other of the nation's news Web sites, was the target of an attack Tuesday that affected access for many of the sites it serves.

The attack on

DoubleClick

marked the third time in two months that hackers have targeted a major commercial Internet service.

Known as a Denial of Service attack, the attack targeted Doubleclick's domain name servers, which translate a domain name like WRAL.com into the numeric IP address needed for Internet connections. Hackers overwhelmed DoubleClick's servers with an abundance of phony Web page requests, blocking many major sites from loading ad images on their sites.

DoubleClick officials said the attack made it difficult for Internet users to load pages at nearly all of the 40 most-visited Web sites, adding that Web pages were available less than 25 percent of the time at one point.

Among the sites hardest hit were WRAL.com,

CNN.com

and

Schwab.com.

Washingtonpost.com

also loaded slowly for several hours Tuesday before it blocked DoubleClick's ads from running on the site.

The FBI said it was not investigating the incident because DoubleClick has not filed a report.

The attack was not related to an incident that happened less than 24 hours earlier, when hackers released a new version of the "MyDoom" worm, which used major search engines to aid in its spread.