On Wall Street in 1920, an explosion on the back of a horse-drawn carriage killed 30 people in an attack blamed on anarchists. Six years before the Sept. 11, 2001, attacks by radical Muslims that killed nearly 3,000 people, there was the Oklahoma City truck bombing led by an anti-government fanatic that killed 168 people. It remains the deadliest incident of domestic terrorism in U.S. history.