Twitter hit by denial of service attack

NEW YORK (CNN) -- The popular social networking site Twitter has been hit by a denial of service attack, according to Twitter co-founder Biz Stone.

In an e-mail to CNN.com, Stone said this morning's attack is not related to a recent incident in which a hacker stole internal documents from the site.

"There's no indication that this attack is related to any previous activities. We are currently the target of a denial of service attack," Stone said in the e-mail.

"Attacks such as this are malicious efforts orchestrated to disrupt and make unavailable services such as online banks, credit card payment gateways, and in this case, Twitter for intended customers or users. We are defending against this attack now and will continue to update our status blog as we defend and later investigate."

Twitter's site went down around 9:30 a.m. ET on Thursday. A message posted on Twitter's status blog said the site was active again by 11:30 a.m., but that the site remained under attack.

"We are continuing to defend against and recover from this attack," the message says.

The site allows users to post 140-character messages, and many of Twitter's users have complained about feeling disconnected from the news of the day while the site is shut down.

In a blog post, Twitter says it will update the public with more information as it becomes available.

A denial of service attack essentially is an attempt to flood a Web site with so much information that it must shut down.