Now radical Muslims around the world protest and promise retaliation for killing Bin Laden. Anger has ignited all across Europe and the Middle East just as the U.S. uncovered a new terror plot being devised by Bin Laden to attack the railways on the anniversary of 9/11.

CAIRO -- Al Qaeda confirmed the killing of Usama bin Laden Friday and vowed revenge, saying Americans' "happiness will turn to sadness" in the first statement by the terror network since its leader was slain in a U.S. commando raid against his Pakistani hideout.

The confirmation came as newly uncovered documents found in bin Laden's residence revealed Al Qaeda plans for derailing an American train on the upcoming 10th anniversary of the Sept. 11 attacks.

Terror experts have said bin Laden's death on Monday was a setback for Al Qaeda but the threat of attacks remains and could even spike in coming days from individuals or small extremist groups inspired to take revenge for the killing.

The statement, dated Tuesday but posted Friday on militant websites, opens the way for the group to name a successor to bin Laden. His deputy Ayman al-Zawahri is now the most prominent figure in the group and a likely contender to take his place.

"The blood of the holy warrior sheik, Usama bin Laden, God bless him, is too precious to us and to all Muslims to go in vain," the statement said. "We will remain, God willing, a curse chasing the Americans and their agents, following them outside and inside their countries."