Federal law enforcement has tracked disparate groups from white supremacists to black militants, eco-terrorists, anarchists and anti-government militias for decades, but some say the attention has been inconsistent. The threat posed by anti-government extremists was brought into focus in the 1990s, when the bombing of a federal building in Oklahoma City and standoffs in Ruby Ridge, Idaho, and Waco, Texas, resulted in bloodshed. But the Sept. 11, 2001, attacks changed the way most Americans think of terrorism.