An important al-Qaida chat forum has been downed by unknown hackers, leaving the organization without a trusted communication channel on the internet.
The Al-Shamukh site appears to have suffered attacks on both its primary web domain and data server.
Two other 'top-tier' jihadi web forums, including the Arabic-language Ansar al-Mujahideen Network, were also mysteriously knocked offline but are now up and running again, says Evan Kohlmann, a counterterrorism expert who tracks jihadist websites for Flashpoint Partners.
"Even with the return of Atahadi and Ansar al-Muj, the ongoing jihadi web blackout is by now the most significant such event since June 10," he says.
"At the present time, Al-Qaida has been left without a trusted operational channel on the Internet for distributing its media and propaganda."
However, Al-Shamukh administrators say they're now working to fix the problem, and claim they'll have it back up and running 'shortly'.
It's not known who brought the sites down. The fact that they've been allowed to continue operating for so long implies that US agencies found it more useful that way.
One obvious suspect is the Jester, a self-styled 'hacktivist for good' who was allegedly behind the hacks of Wikileaks, 4Chan and a number of other Jihadist websites. However, while he reported on his Twitter feed that Al-Shamukh was down, he hasn't claimed responsibility.