Online news sites in the U.K. took a hit Thursday as people hit the Internet for information about the terrorists attacks in London, according to Web performance monitoring companies AlertSite and Keynote Systems Inc.

Four bombs went off Thursday between 8:51 a.m. and 9:47 a.m. London time, crippling the city?s public transport system, killing at least 50 people and injuring hundreds.

The British Broadcasting Corp. experienced the most significant slowdown from 9 a.m. to 1 p.m. London time (5 a.m. and 9 a.m. EDT, respectively), said Alertsite spokesman Ken Godskind. He compared Thursday?s performance of the BBC Web site with its performance during the past week.

?The availability of the BBC site was down to 81 percent, and the response times to load their home page, which normally takes 1 second to load, was as high as 15 seconds on average during that four-hour period,? Godskind said.

CNN was also affected, but not as dramatically. ?The availability of the site was 97 percent, and the time it took to load the home page doubled from 1 second to 1.8 seconds,? he said.

There was a 15 percent increase in the response time for viewing the home page of USA Today, but no change in the response times for ABC News, Fox News or MSNBC, he said.

?We were monitoring London.gov.uk, but it didn?t appear to have any performance issues," Godskind said. "Maybe that?s because it?s not a site where people would go to find updates."

Keynote tracked the U.K. Web sites of Bloomberg.com, the Financial Times and Sky News, said spokeswoman Della Lowe, and at one point, it took over 40 seconds to download the home pages of the Financial Times and Sky News.

From 9:44 a.m. to 1:14 a.m., London time, only about one in four readers could access the Financial Times site, Keynote said. From 10:44 a.m. to 11:44 a.m. London time, the BBC site was also available to only about one in four readers. As for Sky News, it fared even worse: At one point, it was accessible by only about one in six viewers, Keynote said.