No they didn't. They just didn't care, or in some cases were an active part of it. US Police having ties to the KKK and other white supremacist organizations is so supremely not news that it was noted as a growing problem in the Reconstruction.
Sure but klansman cops is more of a local thing, the bigger problem is that looking into right-wing terrorism antagonizes the GOP.
The article is about more recent failings. How work on domestic terrorism almost completely stopped after 9/11/2001.
I get that, but I'm more commenting on the fact that the choice to stop working on domestic terrorism is connected to and linked with the cultural attitudes of both the US in general and the cultural attitudes of the police agencies, both of which are long standing problems that go back before the Civil War. Now, "Choice" is sort of a weird word because in some ways it's less a conscious choice and more one that has a lot of subconscious elements to it, in addition to a level of context within the attitudes and fears of the times. Of course the US was looking outward in the aftermath of 9/11 - Who could blame them for doing that? but the continued focus on that area when there were warning bells going off domestically has a lot to do with those cultural attitudes, and the messaging of all of the state and federal governments since 9/11 (the Obama administration included).