I have long been fascinated by an underground antigovernment movement known as the “sovereign citizens,” who are considered by law enforcement to be on the number one domestic security threat. By Brandon E. Patterson Mother Jones A still from one of Gavin Long’s YouTube video. YouTube On July 17, in the second (at least) targeted attack on police in just over a week, 29-year-old Gavin Long shot six cops, three fatally, in Baton Rouge, Louisiana. The former Marine had posted YouTube selfie videos in which he commented on the need to respond to “oppression” with More…