WASHINGTON — The day before Thanksgiving, President Barack 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 Obama was wrong, but that apparently he was right. By all accounts so far, the government had no concrete intelligence warning of the assault Wednesday that killed 14 people in San Bernardino, California.
Swift, ruthless and deadly, the attack appeared to reflect an evolution of the terrorist threat that 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 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 Obama would address the nation Sunday night about the nature of the terrorist threat and steps the administration is taking to protect the United States. 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 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 "DIY 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 jihadi terrorism on U.S. 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 media and their effect on public fear demand the attention of any administration.
In his weekly radio and Internet address Saturday, Obama warned of the Islamic State's efforts to inspire people in Europe and the United States to carry out attacks.
"We know that ISIL and other terrorist groups are actively encouraging people, around the world and in our country, to commit terrible acts of violence, oftentimes as lone-wolf actors," he said. He urged the country to uphold its values, which administration officials said means not demonizing Muslims.
"We are strong," the president said. "And we are resilient. And we will not be terrorized."
Monaco said the government should be careful not to take actions that feed into the Islamic State's message of Western persecution of Muslims. "ISIL appeals to that and are doing so through social media," she said. "If we do things that play into that, that is letting ISIL win."
Johnson, who has met with Muslims in New York, Chicago, Houston, Los Angeles and elsewhere, said the government needs the Muslim community. "The overarching message to them is, help us help you — help us to identify someone in your community who may be heading in the wrong direction and how can we help you amplify the countermessage to the Islamic State message," he said.
In the case of Syed Rizwan Farook and Tashfeen Malik, the couple identified as the attackers in San Bernardino, all the usual defenses that presidents and counterterrorism officials have built up since Sept. 11 — airport screenings, expansive surveillance, drone strikes — proved no match for a husband and wife assembling weapons of war in the apartment they lived in with their 6-month-old child.
Unlike the perpetrators of the attacks in Paris last month, this couple seemed to have been inspired by the Islamic State but were not acting directly on its orders. On Saturday, law enforcement officers searched the home of a friend believed to have sold two of the weapons used in the massacre, seeking information about whether he had prior knowledge of the attack.
If investigators confirm that the attack was inspired by the Islamic State, it will demonstrate the power of the militant group's message. Through social media, the group can reach past the government's defenses to the nation's heart in encouraging supporters to take up arms in cities, suburbs or small towns.
John P. Carlin, the assistant attorney general for national security, said in an interview Saturday that the Islamic State was adapting. "It's a different iteration of the threat," he said. "Al-Qaida really put a premium on large-scale catastrophic attacks with large loss of life. I think ISIL is trying to explore this as well, but this tactic of small-scale attacks that might fail but still inspire terror" is relatively new for the group.
He added that homegrown terrorists were harder to spot, partly because they act with less preparation. "We used to have a long time from flash to bang because al-Qaida would spend years planning," he said. "Now we see a much shorter time from flash to bang."
James B. Comey, the director of the FBI, had warned in recent months that the Islamic State was seeking to "crowdsource" terrorism by inspiring followers in the West. His agents have been tracking people drawn to the group, but they cannot turn off the Internet in the home of every potential militant.
"When you invest in a narrative, a poisonous narrative that resonates with troubled souls, with unmoored people, and you do it in a slick way through social media, you buzz in their pocket 24 hours a day, saying come or kill, come or kill, that has an impact," Comey said at a news conference Friday.
Federal authorities have charged more than 75 people in cases linked to terrorism in the last two years, about three-quarters linked to the Islamic State and almost all with a social media connection, Carlin said. More than 60 of those were cases related to foreign fighters seeking to join the war in Syria, while about 15 were homegrown extremists. In more than half the cases, the suspects were under 25; in a third of the cases, they were under 21.
What has made the San Bernardino attack all the more alarming is that Farook and Malik tripped none of the usual wires that would alert the authorities. They did not fit the model of the Paris attackers, many of whom were raised in France or Belgium, where Muslims are not as well assimilated economically, politically and socially as they are in the United States. While counterterrorism experts never thought that greater Muslim assimilation in this country meant there could not be an attack here, the assumption that the United States was less vulnerable than Europe has been shaken by San Bernardino.
"The couple was not on any radar and had no real connections to terrorist suspects," said Matthew G. Olsen, a former director of the National Counterterrorism Center. "And what's really troubling is that they appeared to be a well-integrated and stable couple, with a baby and a job."
While it would be a worrisome intelligence failure if the government missed obvious warning signs, William McCants, a former State Department official who worked on countering violent extremism, said the alternative — that there were no signs at all — would be worse.
"It would mean that ISIS fans are learning to be less vocal in their fandom to avoid detection, making them much harder to identify and stop an attack," said McCants, author of "The ISIS Apocalypse," a new history of the Islamic State.
As a result, the massacre may presage a bitter new reality.
"It'll gradually dawn on people," said Bruce Jones, a former U.N. official and the director of foreign policy at the Brookings Institution, "that we'll be living for a long time with the possibility of low-level attacks that can never be predicted and can rarely be prevented."