The document singles out crimes motivated by racial or ethnic hatred, such as the 2019 rampage targeting Hispanics in El Paso, Texas, as well as the threat posed by extremists motivated by foreign terror groups.
The alert comes at a tense time following the riot at the Capitol by supporters of then-President Donald Trump seeking to overturn the presidential election.
Cybersecurity and terrorism expert Alex Crowther, also a research professor at Florida International University, said domestic terrorism isn’t anything new.
Chertoff, who served under President George W. Bush, said attacks by far-right, domestic extremists are not new but that deaths attributed to them in recent years in the U.S. have exceeded those linked to jihadists such as al-Qaida.
“We have to be candid and face what the real risk is,” he said in a conference call with reporters.