A day after Twitter (News - Alert) suffered widespread outages – twice – it’s still not fully clear what caused the problem.

Mazen Rawashdeh, Twitter’s vice president for engineering, apologized and explained that the outages on Thursday – which totaled some two hours – were caused by a “cascading bug” – which wasn’t confined to one software element, but cascaded into other software elements.

“One of the characteristics of such a bug is that it can have a significant impact on all users, worldwide, which was the case today. As soon as we discovered it, we took corrective actions, which included rolling back to a previous stable version of Twitter,” Rawashdeh said in a company blog post.

He flat out denied that the outage was caused by “a hack or our new office or Euro 2012 or GIF avatars, as some have speculated.”

But UGNazi, which is described as a “hacking group,” tweeted that it took down Twitter with a distributed denial of service attack.

Additionally, InformationWeek reported that “Cosmo,” believed to be connected with UGNazi, e-mailed the publication and said, “We just took Twitter.com down with a DDoS Attack.” Following Twitter’s claim the outage came from a bug, Cosmo sent another e-mail to InformationWeek saying, “Twitter moved to multiple servers today to try and migrate [sic] the attack. It was not a bug."

Many on the Web discounted the hackers’ claims. But Reuters (News - Alert) News Service speculated, too, that the alleged attack by UgNazi could have been a “coincidence that coincided with the bug Twitter cited,” according to The Los Angeles Times.

On Thursday, the Twitter site crashed at noon ET and was down until 1:10 pm.  And it crashed again at 1:40 p.m., and was down until 2:08 p.m. ET. During that downtime, Twitter was inaccessible to all Web users and mobile clients were not showing new Tweets, the company said.

The site’s status as of Friday was showing that “the issue had stabilized and all services are restored.” Twitter claims that in the past six months it has had at least 99.96 percent site reliability and stability.

"We are currently conducting a comprehensive review to ensure that we can avoid this chain of events in the future," Rawashdeh concluded in the blog post.

In its report on the Twitter outage, The Wall Street Journal said the company is “one of the next viable candidates for an initial public offering and has recently stressed the reliability of its service, which was prone to outages in its early days.”

In addition, Thursday’s outages demonstrate how important Twitter has become.

Alexander Furnas, a master's degree candidate at the Oxford Internet Institute, wrote on The Atlantic.com that Twitter “has become part of the infrastructure of the social Web. Many people use Twitter as their primary online identity. The increasing interdependence of the social Web, virtually assures that when a service as central as Twitter goes down, ripple effects widespread. I can't tell you the amount, but I guarantee this translates into lost revenue, data, and man-hours for a lot of people.”

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