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In page Hamburg cell:

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"Umar al Masri" turned out to be a nonexistent person. The name was a code word that instructed members of the Taliban office to escort the men to Kandahar, Afghanistan, where they were convinced to join the al-Qaeda network and wage jihad against America. They met with Osama bin Laden himself and swore their loyalty to him. Mohamed Atta was chosen by Bin Laden as the leader of the group that would attack America; Atta would contact Bin Laden several more times before the attacks. The men then returned to Germany to enroll in flight training school, and later moved on to flight training schools in the United States at the recommendation of one of their instructors based in Germany.