WASHINGTON — An elderly man enters a crowded museum carrying a rifle and begins shooting. A young man in Arkansas pulls outside a military recruiting office. Another man opens fire in a Kansas church.

Three chilling, unconnected slayings in less than two weeks. One gunman was a white supremacist, one a militant Muslim, one a fervent foe of abortion.

Each suspect had a history that suggested trouble. Each apparently was driven to act by beliefs considered by some as extreme. Each shooter fits the description of a “lone wolf” terrorist, a killer whose attack, authorities say, is harder to head off than if planned by a trained terrorist network.

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