It seems somebody didn't want to wait for Al Qaeda or Hezbollah or the Nazis but took it in their head to attack right away.
Of course, I'm blogging off the top of my head. I could be wrong - but how likely is that?
Update, 2:48pm: Fox News reports the gunman is a white supremacist. Charles Johnson has excerpts from his website.
You don't just walk into the museum proper with a gun, not the USHMM anyway. The gunman was probably stopped just inside the building at the security desk. Question is, why wasn't he stopped before he got in? This is one of the most heavily-patrolled areas of Washington. There is little parking, so the gunman probably either took a taxi or walked with his shotgun a considerable distance.
Update, 3:04pm: As the alleged gunman describes it,
"In 1981 Von Brunn attempted to place the treasonous Federal Reserve Board of Governors under legal, non-violent, citizens arrest. He was tried in a Washington, D.C. Superior Court; convicted by a Negro jury, Jew/Negro attorneys, and sentenced to prison for eleven years by a Jew judge. A Jew/Negro/White Court of Appeals denied his appeal. He served 6.5 years in federal prison." [courtesy amir/LGF]Pay attention to the word, "treasonous". Not that different from "illegitimate", is it? So the guy thought what he did was legal.
Update, 3:47pm: Illegally parked.
Update, 6/16/09: The real issue, as Richard Cohen puts it, is that
To far more people than we would like to admit, the mystery of James W. von Brunn, the alleged shooter at the Holocaust Memorial Museum, is not that he held such weird and depraved views about Jews and the Holocaust, but that those views are considered weird and depraved...That anti-Semitism is now a part of Middle Eastern culture. It has infiltrated textbooks; it is recited in mosques. It is aired on television -- for instance, the broadcast of a play produced at Gaza's Islamic University in which Jews were portrayed as drinking Muslim blood...James W. von Brunn was quickly segregated from the American mainstream and designated the crackpot he is. In the Middle East, though, he would be no such thing...