Twitter began having serious issues Thursday providing access to those who wanted to shout out a tweet. The popular social site was down for most of the morning. The staff of about 20 posted a note on the "Twitter Status" blog: "We are determining the cause and will provide an update shortly."

It turns out that Twitter has been the target of a distributed "denial of service attack" -- a malicious effort to disrupt and make unavailable services from online banks, credit card payment gateways, and in this case, Twitter for intended customers or users, according to the company's blog. "We are defending against this attack now and will continue to update our status blog as we continue to defend and later investigate," the post reads.

Max Kalehoff, VP of marketing at Clickable, says Twitter has become an important customer outreach and customer service tool. "The site typically has an impressive up time, considering the massive growth and volume it handles," he says, insisting the positive feedback has nothing to do with the fact that Clickable and Twitter share a big investor, Union Square Ventures.

But infusing millions of dollars in the startup can't stop Twitter's mounting problems. The U.S. Marine Corp banned the use of Twitter during duty hours, calling it a security risk because service personnel could inadvertently leak sensitive information.

Twitter also has been hit with a patent lawsuit. TechRadium, a Texas-based firm that developed a mass notification and emergency response service, alleges in the lawsuit that Twitter has infringed on the patent it holds for its IRIS (Immediate Response Information System) emergency notification system.

TechRadium alleges that Twitter is costing the company money, but will license the technology to Twitter for a "reasonable royalty rate," according to court documents obtained by Wired.

The Symantec Security Response team says it monitored the confirmed DDoS attacks Thursday morning on Twitter -- and, yes, Facebook. Symantec has identified a DDoS attack on Facebook, too. People visiting both sites either will notice a slowdown in service or will not have access to the networks.

Marian Merritt, Symantec's Internet Safety Advocate, writes in a blog post that DDoS attacks typically originate from computers infected with bots, turning them into "Zombie computers doing their cybercriminal's bidding." It is not known whether malware is involved, which computers worldwide could have aided in the attack, according to Merritt. Symantec will continue to monitor the situation.

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