A couple of hours ago, the New York Times reported that a terrorist group took credit for the attack:
A terror group, Ansar al-Jihad al-Alami, or the Helpers of the Global Jihad, issued a statement claiming responsibility for the attack, according to Will McCants, a terrorism analyst at C.N.A., a research institute that studies terrorism. The message said the attack was a response to Norwegian forces’ presence in Afghanistan and to unspecified insults to the Prophet Muhammad. “We have warned since the Stockholm raid of more operations,” the group said, according to Mr. McCants’ translation, apparently referring to a bombing in Sweden in December 2010. “What you see is only the beginning, and there is more to come.” The claim could not be confirmed.
This [claim of credit in Arabic] was posted by Abu Sulayman al-Nasir to the Arabic jihadi forum, Shmukh, around 10:30am EST (thread 118187). Shmukh is the main forum for Arabic-speaking jihadis who support al-Qaeda. Since the thread is now inaccessible (either locked or taken down), I am posting it here. I don’t have time at the moment to translate the whole thing but I translated the most important bits on twitter.
Update: Abu Sulayman has now issued a retraction, stating clearly that “Helpers” was not involved in the operation and that his statement was not an official statement. He says those who carried out the attacks “must surely be known to all.”
I assume that Abu Sulayman is implying that al-Qaeda (or an al-Qaeda-linked group) is responsible.