Twitter went down about 9 a.m. Pacific time Thursday, due to a "cascading bug" within one of its infrastructure components, Mazen Rawashdeh, vice president of engineering, . The service had fully recovered at 11:08 a.m. Pacific on Thursday.

"It's imperative that we remain available around the world, and today we stumbled," Rawashdeh said.

A cascading bug refers to a software flaw that causes a chain reaction that takes down other systems. Twitter corrected the problem by rolling back its applications to a previous stable version.

Roughly an hour and a half after the outage started, a hactivist group that calls itself UGNazi, also known as Underground Nazi Hacktivist Group, claimed its DDoS attack took down Twitter. Chester Wisniewski, senior security adviser for Sophos, said he doubted the claim because taking down a global content delivery network like Twitter's would take "tremendous resources."

"During the downtime I had an opportunity to probe the API (application programming interface) and the behavior I observed of certain types of queries intermittently working in waves, but never being totally unavailable, matches nicely with Twitter's explanation as to why they had availability problems," Wisniewski said Friday in an email sent to .