Extremists Emboldened by Capitol Attack Pose Rising Threat, Homeland Security Says

But that warning came in a private channel to law enforcement agencies. Terrorism warnings issued to the public like the bulletin on Wednesday are rare: The most recent came a year ago during a period of tension with Iran after the American military’s killing of Maj. Gen. Qassim Suleimani.

The bulletins issued by the Department of Homeland Security, which was created after the Sept. 11, 2001, attacks, have typically identified foreign terrorist threats. Federal authorities have for years lagged on warnings about the threat of terrorism from within United States borders, perpetrated by American citizens.

“There’s value in soliciting the public’s assistance in identifying and alerting authorities about suspicious activity,” said Brian Harrell, a former assistant secretary for homeland security in the Trump administration. “The watchful public will always be the best ‘eyes and ears’ for law enforcement.”

Asked during a briefing about the motivation for the new terrorism bulletin, Michael Chertoff, a former secretary of homeland security under President George W. Bush, said, “In my view, it is domestic terrorism mounted by right-wing extremists and neo-Nazi groups.” He added, “We have to be candid and face what the real risk is.”

Such candor has long been an exception.

When a warning in a 2009 Department of Homeland Security report, early in the Obama administration, that military veterans returning from combat could be vulnerable to recruitment by terrorist groups or extremists prompted a backlash from conservatives, the homeland security secretary at the time, Janet Napolitano, was forced to apologize.

The report was retracted and an edited version was eventually reissued.

“It was an early lesson in how fraught dealing with these issues can be, but it turns out the report itself and the substance of the report was quite prescient,” Ms. Napolitano said in an interview. “What we saw two weeks ago is what I think we were seeing in 2009, but it has only grown and it seems to have exploded in the last four years.”

This week, Mr. Biden ordered a comprehensive assessment of the threat of domestic violent extremism. During his confirmation hearing, the president’s pick for homeland security secretary, Alejandro N. Mayorkas, said he would empower the department’s intelligence branch, which has long struggled to distinguish its assessments from the F.B.I.