(CNN) -- An attack on the social networking site Twitter shut the site down for about two hours on Thursday morning, causing headaches in the online community and glitches in other Web sites like Facebook.
In an e-mail to CNN.com, Twitter co-founder Biz Stone said the site was hit with a "denial of service attack," or an attempt to shut the site down by overwhelming it with traffic.
"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.
Users of other social networking sites were also experiencing trouble on Thursday morning. Facebook users reported pieces of the site were down and there was speculation those problems were related to the Twitter attack.
"We're looking into it and will give you an update as soon as possible," Facebook's Brandee Barker told CNN.
Twitter 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 offline.
Although its status blog claimed Twitter was back online, users were still encountering problems in accessing the micro-blogging site.
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.