US Attorney General Merrick Garland announced a crackdown Monday on threats against schools and teachers after a surge of verbal attacks by parents opposed to mask and vaccine mandates and education on race bias.

"In recent months there has been a disturbing spike in harassment, intimidation and threats of violence" against teachers, school administrators and other staff, Garland said in a memo to the Justice Department and FBI.

"Threats against public servants are not only illegal, they run counter to our nation’s core values," Garland said.

"The department takes these incidents seriously and is committed to using its authority  and resources to discourage these threats, identify them when they occur, and prosecute them when appropriate," he said.

Garland did not mention what was driving the spike, and said he respected "spirited debate."

But the memo came after dozens of incidents across the country in which irate parents -- who object to mandates for student masking, vaccine requirements and teaching children about structural racism in society -- have been seen threatening school boards, teachers and school principals.

Last week the National School Boards Association called on President Joe Biden to intervene after a surge in threats, many seen on viral videos taken at community meetings.

"America’s public schools and its education leaders are under an immediate threat,” said NSBA President Viola Garcia and CEO Chip Slaven in a letter to Biden.

"The National School Boards Association respectfully asks for federal law enforcement and other assistance to deal with the growing number of threats of violence and acts of intimidation occurring across the nation," they said.

They detailed numerous violent threats and physical attacks, by parents angered by Covid-19 policies and opposed to what they wrongly believe is primary and secondary schools teaching "critical race theory," an approach to social justice studies mostly taught at the university level. 

"As the threats grow and news of extremist hate organizations showing up at school board meetings is being reported, this is a critical time for a proactive approach to deal with this difficult issue," they said.

A similar statement was issued by the School Superintendents Association on September 22.