Twitter was unavailable for about two hours Thursday due to a bug or to a denial of service attack.

Twitter claims the problem was equipment-related. "Today's outage is due to a cascaded bug in one of our infrastructure components," a company spokesman said in an email, denying the involvement of hackers.

The hacking group UGNazi, however, claimed via Twitter to have been responsible for taking down Twitter.

In a separate email sent to InformationWeek, a person claiming to be Cosmo, one of the individuals associated with UGNazi, reiterated that claim. "Hello, I am Cosmo from UGNazi and I would like to inform you we just took twitter.com down with a DDoS Attack. It has been down for 20 minutes now," the email said.

[ Read Social Game Maker Stops HTML5 Project. ]

The person claiming to be Cosmo disputed Twitter's denial in a follow-up email. "Twitter moved to multiple servers today to try and migrate [sic] the attack," the sender said. "It was not a bug."

Thursday afternoon, Twitter posted more information about the outage.

"At approximately 9:00am PDT, we discovered that Twitter was inaccessible for all Web users, and mobile clients were not showing new Tweets," Mazen Rawashdeh, Twitter's VP of engineering, said in a blog post. "We immediately began to investigate the issue and found that there was a cascading bug in one of our infrastructure components. This wasn't due to a hack or our new office or Euro 2012 or GIF avatars, as some have speculated today."

Yet asked directly whether UGNazi's claims were inaccurate, a Twitter spokeswoman replied, "We don't have a comment on that."

So could those speaking for UGNazi be telling the truth? Possibly. A DDoS attack, after all, isn't technically a hack. No vulnerabilities or exploits affecting Twitter's systems are involved in sending a flood of data to overwhelm Twitter's servers. Pressed for further clarification, Twitter did not immediately respond.

Cosmo, identified as "Hannah Sweet" on the UGNazi website, was arrested by the FBI in May. The FBI did not immediately respond to a query submitted to its national press office about whether Cosmo could have been involved in the supposed attack on Twitter.

The person claiming to be Cosmo confirmed being arrested by the FBI but, in an email, declined to discuss the matter.

Cosmo's arrest was reportedly related to UGNazi's alleged involvement in the breach of billing company WHMCS in May. UGNazi also claimed responsibility for hacking a Web service called CloudFlare earlier this month in order to conduct an attack on 4Chan.

Though Twitter was accessible Thursday evening, the company's status blog indicated ongoing issues five hours after the initial problem was reported. Twitter has reported far fewer site reliability issues in 2012 than in previous years.

According to Rawashdeh, Twitter has enjoyed its highest reliability to date over the past six months.

Black Hat USA Las Vegas, the premiere conference on information security, features four days of deep technical training followed by two days of presentations from speakers discussing their latest research around a broad range of security topics. At Caesars Palace in Las Vegas, July 21-26. Register today.