Der Spiegel is reporting that a group of radical islamists is claiming responsibility:
A group calling itself “Secret Organization ? al-Qaida in Europe” has posted a claim of responsibility for the series of blasts in London, a German magazine reported Thursday.
Der Spiegel magazine reported that the group posted its message on a Web site popular with Islamic militants, which it did not name. It said the group claimed the explosions were in retaliation for Britain’s involvement in Iraq and Afghanistan.
Update (by Nick at 1pm): Just to let folks know that myself and Matt Turner are fine, and it seems so far that various London bloggers have avoided being caught up in it. Until last week, I had to travel through Liverpool Street station on a regular basis, and Matt normally passes through Edgware Road station every day.
Update 14:15 UK time.
Associated Press Cairo office continue the story, elaborating a bit more. The caim contains details worthy of note insofar as it mentions Iraq *and* Afghanistan (Qaeda and Taliban activity seems to have been increasing of late) and secondly it directly threatens Denmark and Italy. The second source citing this may be more significant than Der Spiegel since it is Elaph, a secular Arab-language news website. Elaph suggests that the site were the claim was posted is one typically used by islamic militants. Of course this confirms nothing. It is simply one possible lead, the only one we have to date.