The suspect is a Holocaust denier who once tried to kidnap members of the Federal Reserve Board (something he calls "citizen's arrest"). James Von Brunn managed to kill a guard before being shot himself, and is currently in critical condition.
He is described as a "frustrated artist," who angrily stomped out of a gallery that wouldn't show his work. On his website he describes a childhood bully, service in World War II and many problems he blames on Jews and other minorities.
Because he was caught early in his rampage, and is currently in no condition to explain himself, it's difficult to tell exactly what Von Brunn's motives were. His rhetoric and choice of location suggest an ideologically motivated assault, but, going into a federal museum with little effort to hide his weapons, and killing the first person he encountered with security everywhere guaranteed that he would be caught almost immediately. The sheer amount of his arms also guaranteed that security would feel impelled to use force, so was this an attempted "suicide by cop," as many mass murders have ended? In addition to racist anger that the Jews are "undermining" the white race with the "browning of America," Von Brunn's website describes various perceived personal hardships, the kind that can lead an unstable mind to mass murder culminating in suicide. I think psychologists have said that this level of hatred in an individual can be traced to a projection of self-loathing, at least in some cases. But Von Brunn isn't talking right now, and at the moment, all we have to go on is his self-aggrandizing website and the details of an appalling but poorly executed crime.