Unfortunately, the US govt's recent toleration of right-wing terrorists (while officially working to oppose them), is nothing new. Look back at the numerous ways in which the FBI supported KKK terror against the Southern Civil Rights movement, including involvement in the infamous Birmingham church bombing.

This essay by Howard Zinn is one of many documenting the FBI's nefarious role in repressing the Civil Rights movement:

http://mediafilter.org/caq/FBI.html

I also recommend Brian Glick's work on the FBI:

http://www.thirdworldtraveler.com/FBI/COINTELPRO60s_WAH.html

Here is an excerpt from Glick's essay:
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7) "White Hate Groups" (1964-71): This unique "program" functioned largely as a component of the FBI's operations against the progressive activists who were COINTELPRO's main targets. Under the cover of being even-handed and going after violent right-wing groups, the FBI actually gave covert aid to the Ku Klux Klan, Minutemen, Nazis, and other racist vigilantes. These groups received substantial funds, information, and protection-and suffered only token FBI harassment-so long as they directed their violence against COINTELPRO targets. They were not subjected to serious disruption unless they breached this tacit under standing and attacked established business and political leaders.
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Also, check out this movie about COINTELPRO, with a good chunk focusing on FBI support for the KKK: