Reddit mentions of The FBI and the KKK: A Critical History

The law has nothing to do with it, the law is just some words. The reality of how the FBI operates is the pertinent question.

They have consistently chosen not to investigate the KKK preferring instead to infiltrate Black and socialist groups.

They murdered Fred Hampton for example

Hi bizzle. While there are numerous histories of the FBI, varying from the most obsequious and hagiographical to more objective and critical analyses based on the actual documented record, one recent history worth considering is "The FBI and the KKK: A Critical History", the subject matter of which is evident from the title: "The Federal Bureau of Investigation and the Ku Klux Klan share a long and complicated history. Beginning with their first confrontation in 1922, this book examines the similarities, covert collaborations and common goals of the FBI and the KKK. After briefly describing the history of each, it explores the development of their association and the specific ways in which each organization furthered the other's goals. The book traces eighty years of parallel development and the conservative attitudes that, astonishingly, drew the FBI and the KKK together."

But we can go back further than that analysis, back to the FBI's predecessor, the BOI (Bureau of Investigation), which underwent a massive expansion in 1910 in order to implement in a perversely over-zealous way, the sexist and racist and bizarrely titled "White Slave Act" (otherwise known as the Mann Act of 1910). In addition to its implicit racism (ignoring the pervasive black enslavement at that time), it was actually about a hysteria in relation to women gaining more independence and autonomy after the Suffragettes, an attempt to confine white women to domestic enslavement (as with blacks), independent women being dismissed as "loose women" or worse, criminalized as prostitutes. The expansion of a fledgling FBI led to the psychotic implementation of that Act's provisions with a demented sexist zeal. And all this even before the 48 years rule of the notoriously reactionary and paedophilic J Edgar Hoover arrived on the scene in the 1920s.

As for the FBI's relationship to the racist far right and the KKK, that is continuing right up to the present day, as this very recent article makes clear:
"Revealed: FBI investigated civil rights group as 'terrorism' threat and viewed KKK as victims

Bureau spied on California activists, citing potential ‘conspiracy’ against the ‘rights’ of neo-Nazis":

https://www.theguardian.com/us-news/2019/feb/01/sacramento-rally-fbi-kkk-domestic-terrorism-california
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Going back further again in history, the entirely private agency that was the proto-type of the FBI was the mobster-linked and classist/anti-union cabal of gangsters called the private Pinkerton Detective Agency that originated in the mid-19th century. Government, government and state agencies, and private corporations, industrialists, oligarchs, hired Pinkerton (it still exists today but is called Securitas AB) as armed thugs to spy on and infiltrated unions and worker organisations (recall John Sayles' film Matewan that portrays a well-known confrontation between Pinkerton thugs and workers)