An 88-year-old white supremacist with a rifle walked into the United States Holocaust Memorial Museum, one of the capital’s most visited sites, on Wednesday afternoon and began shooting, fatally wounding a security guard and sending tourists scrambling before he himself was shot, the authorities said.

The gunman was identified by law enforcement officials as James W. von Brunn, who embraces various conspiracy theories involving Jews, blacks and other minority groups and at one point waged a personal war with the federal government.