Both the Anti-Defamation League and the Southern Poverty Law Center had tracked [the Sikh Temple shooter] Page for several years. The nonprofit organizations collect publicly available information on hate groups from Web forums, pamphlets and other sources.
But the FBI is prohibited under federal law from collecting information on U.S. citizens not suspected of committing a crime. In order to open a domestic terrorism investigation, FBI agents must believe a suspect has threatened violence, has broken federal law and is trying to advance a political or social agenda.
This sets the bar high for opening a domestic terrorism case before someone has made a specific threat of violence or committed a crime.