In their heyday during the 1990s, domestic extremist groups created spectacular violence -- the explosion at the federal building in Oklahoma City and standoffs with law agencies at Ruby Ridge, Idaho, and Waco, Texas.

Some say the Sept. 11, 2001, terrorist attacks diverted homespun rage against the U.S. government and toward foreigners, the Times reported Tuesday. Others say the movement began to fade when the Y2K disaster -- a favorite prediction of conspiracy theorists -- failed to occur.

Human nature also played a role.

"Many of the people had such huge egos that they didn't know how to work together and keep the movement going," Chip Berlet, a senior analyst at the Political Research Associates think tank, told the Times. "So it basically unraveled."

The U.S. Justice Department compiled a summary on foreign and domestic terrorism for 2002-05, finding 23 of the 24 attacks committed by domestic groups were by "special-interest extremists active in the animal-rights and environmental movements." The other was a synagogue firebombing in Oklahoma City by a white supremacist.