Perhaps most disturbing of all, both groups have real-world war zones in which to learn combat. Jihadis had Afghanistan in the 1980s, the Balkans in the 1990s and Syria today. White supremacists have the war in eastern Ukraine, in which they are fighting on both sides. Dr. Kacper Rekawek, a scholar who has studied the matter, estimates that 17,000 people from 50 countries, including the United States and many of its allies, have traveled to fight in Ukraine. Those with ties to far-right militias in Ukraine include at least one of four Americans indicted on a charge of promoting the deadly violence at the Unite the Right rally in Charlottesville, Va., in 2017. The New Zealand mosque attacker claimed in his manifesto that he had traveled to Ukraine. What we know for sure is that during his attack he wore a flak jacket bearing a symbol of one of the country’s main ultranationalist groups.

Against this backdrop, it is hardly surprising to see the white-supremacist threat growing inside the United States. A study by the Anti-Defamation League found that, in 2018, right-wing extremists were responsible for three times as many deaths in the United States as were Islamists. The same study showed that 2018 was the deadliest year of right-wing extremist violence since 1995 — when the Oklahoma City bombing took place. Because of massacres like the one on Saturday in El Paso, the year 2019 may yet prove worse.

Our intelligence and law-enforcement agencies are not blind to the threat. In May, a senior F.B.I. official testified to Congress that the bureau is pursuing about 850 domestic terrorism investigations. But our current counterterrorism framework was set up, in the immediate aftermath of 9/11, to deal exclusively with foreign terrorist groups like Al Qaeda. For example, the law allows for the monitoring of communications between people connected with foreign terrorist groups — even if they are United States citizens operating on American soil — and the sharing of the resulting intelligence among American agencies and with our allies. But those monitoring and intelligence-sharing tools cannot be used against those connected with terrorist groups based in the United States — no matter how dangerous — because domestic terror supporters are protected by free speech laws in ways that jihadis (including those who are United States citizens) are not.

Since 2001, a long list of people have been indicted on a charge of providing material support to designated foreign terrorist entities like Al Qaeda. But for domestic terrorist organizations, material support charges are impossible because there is no mechanism for designating domestic terrorist groups as such. Moreover, domestic terror charges are harder to prove and carry penalties inadequate to the gravity of the offense. Even the Oklahoma City bomber, Timothy McVeigh, the worst domestic terrorist in the nation’s history, was not charged with any terrorism offense for precisely this reason.