Last week, federal agents in Maryland arrested a United States Coast Guard officer and said he was plotting to assassinate Democratic members of Congress, prominent television journalists and others. The officer, Lt. Christopher Hasson, apparently inspired by a right-wing Norwegian terrorist who slaughtered 77 people in 2011, stockpiled firearms and ammunition and researched locations around Washington to launch his attacks, according to investigators. Fortunately, the F.B.I. arrested him before he could act.

This frightening case is just one of several recent reminders that white supremacy and far-right extremism are among the greatest domestic-security threats facing the United States.

Regrettably, over the past 25 years, law enforcement, at both the federal and state levels, has been slow to respond. This is in part because of the limited number of enforcement tools available to prosecutors. But there are steps that can be taken to help the police and prosecutors address this growing threat — including, on the federal level, a domestic terrorism law.