Al-Mauretani had decided the best scheme would be to launch a suicide bombing on a train as it entered a tunnel. And he told Vinas that preferably a white operative with Western travel documents would be tasked to carry out the attack.

In 2010, al-Mauretani was seen as the mastermind of planned attacks in Europe. Fears that such attacks would materialize led the U.S. State Department to issue a travel alert in October 2010.

Sources briefed on the letter told CNN that al-Mauretani requested bin Laden issue a statement saying al-Qaida's patience with Europe had run out following the al-Qaida leader's previous offering of a conditional truce, and that his statement needed to be choreographed with an attack shortly afterwards.

"We ask you undertake certain steps in order to threaten Europe before the attacks happen. And these steps should be in synch with the preparations of those attacks. Inform Europe that patience has come to an end, as has our hope that they end their campaign against us. Also (make clear) that they have not understood our message thus far. One or two weeks after that we will strike ... and then we will threaten them again. After we hit Europe we will hit America, so we isolate the Americans," the sources said al-Mauretani wrote in the letter.

Die Zeit's Musharbash says al-Mauretani's blueprint "has very likely little operational value now. But certain ideas may have trickled down and may still be alive elsewhere in the network."