It’s been more than three weeks since the Pittsburgh synagogue shooting. But there are still a lot of lingering questions. One of them is whether the gunman had links to Atomwaffen, a neo-Nazi group that calls for racially motivated, lone-wolf terror attacks like the one in Pittsburgh. It also recruits inside the U.S. military. A new PBS documentary takes a closer look at the group. “Documenting Hate: New American Nazis” airs on PBS’s Frontline tonight.