 Blowback, or unintended consequences of funding the mujahideen, was said to have come to the United States in the 1993 World Trade Center bombing and the September 11 attacks.In the 1993 bombing, all of the participants in the bombing "either had served in Afghanistan or were linked to a Brooklyn-based fund-raising organ for the Afghan jihad" that was later "revealed to be al-Qaeda's de facto U.S. headquarters".Principals in the 2001 attack—Osama Bin Laden, Khalid Sheikh Mohammed – had both fought in Afghanistan, and bin Laden was a lieutenant of Abdullah Azzam.