Al-Qaida's main Web forums have been offline for the past 11 days in what experts say is the longest sustained outages of the sites since they began operating eight years ago.
No one has claimed responsibility for disabling the sites, but the breadth and the duration of the outages have prompted some experts to conclude the forums have been taken down in a cyberattack, launched perhaps by a government, a government-backed organization or a hacking group.
The first website, Shumukh al-Islam, a primary source for al-Qaida videos and messages, went down March 22, and since then four others have gone dark. The administrator of a second-tier al-Qaida site recently posted a message on an online forum saying that "the media arena is witnessing a vicious attack by the cross and its helpers on the jihadi media castles."
Officials in the United States and elsewhere have long been concerned about sites associated with al-Qaida. Those sites have been used to call for violence against Western targets and to try to recruit Islamic extremists to carry out attacks.
There is uncertainty over whether the recent outages were caused by a cyberattack, and some skeptics note that some prominent al-Qaida forums remain online. U.S. government agencies, including U.S. Cyber Command, had no role in the outages, according to officials who would speak only on condition of anonymity.
Still, Will McCants, a former State Department counterterrorism official who is now a senior fellow at the Homeland Security Policy Institute at George Washington University, said given the number of sites down and the duration of the outages, "it sure looks like a takedown."
If it were a technical problem being addressed by site administrators, "usually they will get on another site and say we've got administrative problems," McCants said.
The last lengthy blackout of al-Qaida Web forums took place in summer 2010, when British intelligence officials disrupted the launch of an online magazine produced by the network's affiliate in Yemen.
In that case, the most prominent al-Qaida site at the time, al Fallujah Web forum, was dark for at least seven days, said Evan Kohlmann, senior partner at Flashpoint Global Partners, which tracks the sites, which are mostly in Arabic language.
Although he generally sees the disruption of al-Qaida websites as a fruitless game, Kohlmann said the most recent outages have clearly begun to affect jihadi communications.
"At least temporarily, the social networking among jihadists has been disrupted," he said. "The remaining forums are really struggling to attract the participation of users."