U.S. faces heightened threats from violent domestic extremists after Capitol attack, Homeland Security says.

Warning that the deadly rampage at the Capitol this month may not be an isolated episode, the Department of Homeland Security on Wednesday said publicly for the first time that the United States faced a growing threat from “violent domestic extremists” emboldened by the attack.

The department’s terrorism alert did not name specific groups that might be behind any future attacks, but it made clear that their motivation would include anger over “the presidential transition, as well as other perceived grievances fueled by false narratives,” a clear reference to the accusations made by President Donald J. Trump and echoed by right-wing groups that the 2020 election was stolen.

“D.H.S. is concerned these same drivers to violence will remain through early 2021,” the department said.

The Department of Homeland Security does not have information indicating a “specific, credible plot,” according to a statement from the agency. The alert issued was categorized as one warning of developing trends in terrorism, rather than a notice of an imminent attack.

But an intelligence official involved in drafting Wednesday’s bulletin said the decision to issue the report was driven by the department’s conclusion that Mr. Biden’s peaceful inauguration last week could create a false sense of security because “the intent to engage in violence has not gone away” among extremists angered by the outcome of the presidential election.

The warning contained in a “National Terrorism Advisory System Bulletin” was a notable departure for a Department of Homeland Security accused of being reluctant during the Trump administration to publish intelligence reports or public warnings about the dangers posed by domestic extremists and white supremacist groups for fear of angering Mr. Trump, according to current and former homeland security officials.

Starting with the deadly extremist protest in Charlottesville, Va., in 2017, when Mr. Trump said there were “very fine people on both sides,” he played down any danger posed by extremist groups. And when racial justice protests erupted nationwide last year, his consistent message was that it was the so-called radical left that was to blame for the violence and destruction that had punctuated the demonstrations.

Even after the Department of Homeland Security in September 2019 singled out white supremacists as a leading domestic terrorism threat, analysts and intelligence officials said their warnings were watered down, delayed or both. Former officials in the Trump administration have even said that White House officials sought to suppress the phrase “domestic terrorism.”

The intelligence official involved with the bulletin, who spoke on the condition of anonymity to discuss its findings, added that the public warning should have been issued as early as November, when Mr. Trump was making an escalating series of false accusations about the election, and that far-right groups continued to be galvanized by such false statements.

But at the time, Mr. Trump was also seeking to dismiss department officials whom he regarded as disloyal, including Christopher Krebs, the chief of its cybersecurity agency, after a committee overseeing the election declared it had been “the most secure in American history.” The agency failed to issue a warning to state and local agencies warning of specific violence aimed at the Capitol before the attack on Jan. 6.