Der Spiegel reports:

A letter posted on a Web site frequently used by al-Qaida claims that the “Secret Organization of al-Qaida in Europe” is responsible for today’s massive terrorist attack in London. In it, the group also issues warnings to Denmark and Italy.

In a letter located by SPIEGEL ONLINE on a Web site trafficked by al-Qaida, a document that claims to be written by the interntional terror group claims responsibility for the London attacks.

“Rejoice, community of Muslims,” the letter states. “The heroic mujahedeens today conducted an attack in London,” it continues. All of Great Britain is now shaken and shocked, “in the north, the south, west and east.” “We’ve warned the British government and the British people time and again,” the letter adds. “We’ve kept our promise and have carried out a blessed military operation.”

“We continue to warn the governments of Denmark and Italy and all other crusader governments.” We demand that all countries pull their troops out of Afghanistan and Iraq, states the letter, which has been signed by the “Secret Organization — al Qaida in Europe.”

The authenticity of the document could not be immediately confirmed…

Reuters: It’s a classic al Qaeda attack.

Apparently coordinated blasts across London’s transport network on Thursday bear similarities to last year’s Madrid bombings and suggest an attack in the style of al Qaeda, security analysts said.

Officially, there was no immediate confirmation from British authorities that they were treating the explosions as an attack by militants. London’s police chief said there were about six blasts.

The use of near-simultaneous attacks to cause maximum damage and panic is a tactic frequently used by al Qaeda, from the 1998 bombing of two U.S. embassies in East Africa to the Sept. 11 attacks on the United States with four hijacked airliners in 2001.

Since then, the pattern has been repeated in deadly attacks attributed to al Qaeda or like-minded Islamist militants in Indonesia, Kenya, Morocco, Saudia Arabia, Turkey and Spain.

At The Counterterrorism Blog:

Victor Comras notes that the trial of Abu Mazra al-Masri, Britain’s best-known Islamic radical, started in London two days ago on multiple charges of soliciting the murder of others. Walid Phares noted on NBC that several groups threatened attacks in Britain in recent months.

Update: Evan Kohlmann at CT thinks the Internet claim is likely a hoax.

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Cross-ref’d: The 7/7 attacks: Britain’s breeding ground for suicide bombers