WASHINGTON — The day before Thanksgiving, President Obama reassured Americans there was “no specific and credible intelligence indicating a plot on the homeland.” Seven days later came an explosion of gunfire and the deadliest terrorist attack in America since Sept. 11, 2001.

What may be most disturbing is not that Mr. Obama was wrong, but that apparently he was right. By all accounts so far, the government had no concrete intelligence warning of the assault on Wednesday that killed 14 people in San Bernardino, Calif.

Swift, ruthless and deadly, the attack appeared to reflect an evolution of the terrorist threat that Mr. Obama and federal officials have long dreaded: homegrown, self-radicalized individuals operating undetected before striking one of many soft targets that can never be fully protected in a country as sprawling as the United States.

“We have moved to an entirely new phase in the global terrorist threat and in our homeland security efforts,” Jeh Johnson, the secretary of Homeland Security, said in an interview on Saturday. Terrorists have “in effect outsourced attempts to attack our homeland. We’ve seen this not just here but in other places. This requires a whole new approach, in my view.”

The White House announced that Mr. Obama would address the nation on Sunday at 8 p.m. Eastern time about the nature of the terrorist threat and steps the administration is taking to protect the United States. Mr. Johnson said the government should continue to augment airline security by placing more agents in overseas departure airports and further toughen standards for the visa waiver program that allows visitors from certain friendly nations easy entry into the country. He and other officials said the government needed to reach out even more to Muslim communities to help identify threats that might otherwise escape notice.

Unable to curb the availability of guns at home or extremist propaganda from overseas, the authorities may have to rely more on encouraging Americans to watch one another and report suspicions. Federal and local governments already have programs urging friends, families and neighbors to identify people targeted for recruitment.

The attack may reignite the privacy-versus-security debate about encryption software sold by private-sector providers over government objections. And some administration officials said they needed to escalate efforts to stimulate contrary Muslim voices to counter extremist propaganda by the Islamic State, also known as ISIS or ISIL.

“We can work with the private sector to get additional messengers with alternative voices out there,” said Lisa Monaco, the president’s counterterrorism adviser. “Frankly, we’ve got to do a better job of approaching this in a way that allows us to — the phrase has been used — break the brand of ISIL’s message.”

The San Bernardino attack has already inflamed the political debate less than two months before the first voting in the 2016 presidential primaries, and it may reshape Mr. Obama’s last year in office. While Republican candidates denounced the president, politicians were not the only ones asserting that his administration should shift course.

John D. Cohen, a professor at Rutgers University and a senior Homeland Security Department counterterrorism official until last year, said the administration needed to “wake up” to the threat and change an approach that is “ill-suited to deter these kinds of attacks.”

Alberto M. Fernandez, who until earlier this year led the State Department unit that counters militant propaganda, said, “The administration seems to be really flailing and tone deaf to this latest challenge.” He called the San Bernardino attack “D.I.Y. jihad,” and said it “forces the administration to look at where it does not want to go and is weakest, at jihadist ideology and its dissemination.”

Others, however, cautioned against overreaction, warning that the focus on Muslims could lead to the kind of anger and alienation that creates more potential for terrorist recruitment. Some experts urged officials to keep the danger posed by terrorism in perspective.

The death toll from jihadist terrorism on American soil since the Sept. 11 attacks — 45 people — is about the same as the 48 killed in terrorist attacks motivated by white supremacist and other right-wing extremist ideologies, according to New America, a research organization in Washington.

And both tolls are tiny compared with the tally of conventional murders, more than 200,000 over the same period. But the disproportionate focus they draw in the news media and their effect on public fear demand the attention of any administration.

In his weekly radio and Internet address on Saturday, Mr. Obama warned of the Islamic State’s efforts to inspire people in Europe and the United States to carry out attacks.