This is a fake study. The author ("jdje") took a report from the left-leaning Center for Strategic and International Studies (CSIS), which concluded (probably wrongly) that right-wing terrorism is a greater threat than left-wing terrorism, and he edited it to change "right" to "left" and "white supremacists" to "black supremacists," to invert the meaning. He also replaced the CSIS report's references with some conservative references at the end. Here's the original CSIS report: