The shootings in Texas and Ohio that killed at least 31 people over the weekend left authorities searching for how to confront the challenges posed by mass violence and domestic terrorism, especially attacks driven by white-nationalist ideologies.

Violence committed by white men inspired by an extremist ideology make up a growing number of domestic terrorism cases, according to the Federal Bureau of Investigation. Of about 850 current domestic terrorism cases, 40% involve racially motivated violent extremism and a majority of those cases involve white supremacists, the FBI said.

Saturday’s attack in majority-Hispanic El Paso, Texas, which left at least 22 people dead, was allegedly committed by a 21-year-old white man who was believed to have posted a manifesto of sorts that espoused anti-immigrant and white-nationalist ideology on a popular far-right website not long before the shooting.

Assailants in other recent attacks, including at synagogues in Pittsburgh and Poway, Calif., also espoused white-nationalist beliefs.

“We are most concerned about lone offenders, primarily using firearms, as these lone offenders represent the dominant trend for lethal domestic terrorists,” Michael McGarrity, the FBI’s top counterterrorism official, recently told lawmakers. “Frequently, these individuals act without a clear group affiliation or guidance, making them challenging to identify, investigate and disrupt.”