Mr. Herdman’s office has charged others with making threats against federal law enforcement officers and against Representative Alexandria Ocasio-Cortez, Democrat of New York and a frequent target of conservative criticism.
Those arrests came after a man opened fire in August in a nightclub on the other side of the state, in Dayton, gunning down nine people and wounding 19. The gunman possibly embraced troubling beliefs, including anti-government, racist and misogynist views, according to a law enforcement official.
Civil liberty and Muslim advocacy groups have accused the government of being slow to recognize the deadly threat as investigators focused heavily on Islamic terrorists.
“For too long, the F.B.I. was myopically targeting Muslims as potential terrorists,” said Faiza Patel, co-director of the Liberty and National Security Program at the Brennan Center for Justice at the New York University Law School. “It is now feeling pressure from Congress and the public to address white nationalist violence, so we are seeing a wave of investigations and prosecutions.”
Around the country, federal law enforcement officials have vocally taken on domestic terrorism. Thomas T. Cullen, the top prosecutor in the Western District of Virginia and a Trump appointee, has moved aggressively to convict white supremacists who break the law.
He prosecuted James Fields Jr., the avowed white supremacist from Ohio who steered his Dodge Challenger into a crowd of protesters near a white nationalist rally in Charlottesville, Va., in 2017, killing a young woman and injuring dozens. As Mr. Fields was sentenced in June to life in prison, Mr. Cullen said his attack was “motivated by this deep-seated racial animus.”
Mr. Cullen has targeted local members of a Southern California-based violent white supremacist group, the Rise Above Movement. Two regions of growing concern are the West Coast and the states around the Great Lakes, where the F.B.I. has seen more arrests than in other parts of the country.