This week’s mass shooting in San Bernardino, Calif., is prompting new fears about terror attacks inside the United States, and shows the limits of federal efforts to halt all instances of extremist violence.

The government has devoted massive resources to halting such attacks from Islamic militants in the years since 9/11, but the very nature of “lone wolf” radicals makes them nearly impossible to prevent.

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Wednesday’s episode underscored the unpredictability of the threat facing the country, analysts said. A recently married couple with a newborn baby, no history of radicalization and family connections to a decorated U.S. military veteran apparently carried out what is being investigated as the deadliest attack on American soil since 9/11.

The shooting will inflame anxiety about terrorism from groups including the Islamic State in Iraq and Syria (ISIS) — worries that had already spiked following last month's attacks in Paris in which 130 people were killed. Yet the government's ability to respond may be limited.

“The lone wolf problem is so incredibly difficult to predict, because anyone can, again, wake up in the morning and pledge allegiance to ISIS without ever having direct contact with a terrorist, and then go out and commit an act of violence in the name of that group,” said Rick Nelson, a former official with the National Security Council and National Counterterrorism Center who is now a senior non-resident associate at the Center for Strategic and International Studies.

“How do you stop that?”

The two suspects behind Wednesday’s attack, Syed Farook and Tashfeen Malik, were not on any government watch list, officials say. Evidence collected by the FBI after the couple was killed in a shootout with police suggests they were not connected to ISIS or any other network or local cell, Director James Comey said on Friday.

Farook was a U.S. citizen and his wife came to the country as his fiancee, so neither would have been impacted by Congress's recent focus on refugees and foreign tourists.

Nor did they seem to have any clear personal or political motivations to carry out such a deadly attack.

Farook’s brother reportedly served four years in the U.S. Navy and left the service with multiple medals to his name. The two apparent attackers had a daughter who was just 6 months old, who was left with her grandmother on Wednesday under the pretense that her parents had a doctor’s appointment.

“We didn’t know,” David Bowdich, the assistant director of the FBI's field office in Los Angeles, told reporters on Friday. “There’s nothing that we’ve seen yet that would have triggered us to know.”

Throughout the course of their investigation, however, federal officials are starting to see the breadcrumbs of the couple’s apparent self-radicalization.

Shortly ahead of the attack on Wednesday, Malik is believed to have posted a Facebook message pledging allegiance to ISIS leader Abu Bakr al-Baghdadi.

Farook was also reportedly in contact with terror suspects known to the federal government. But the FBI has dismissed any speculation that that should have caused them to take notice.

“I would urge you not to make too much of that,” Comey said on Friday.

The couple possessed what is being described as a massive arsenal and appear to have attempted to erase their digital footprints, a detail that suggests advanced planning.

During their investigation, officials are attempting to restore that deleted information, which could offer revealing clues about the couple’s path to violence.

Investigators are also trying to glean more information about Farook’s 2013 trip to Saudi Arabia, during which time he met Malik and participated in a pilgrimage that all Muslims are expected to take once in their lives.

“What they did when they were over there, that’s what we don’t really know right now,” Rep. Michael McFaul (R-Texas), the head of the House Homeland Security Committee, told The Hill this week. “That’s what we’re investigating.”

Wednesday’s massacre in California was at least the third lone-wolf style attack from apparent Islamic extremists on U.S. soil this year, following a May shooting at an event in Garland, Texas, featuring cartoons of the Prophet Mohammad and a July attack on military facilities in Chattanooga, Tenn.

There have been other attempts, according to federal law enforcement officials.

The FBI has carried out investigations in all 50 states, and 71 people have been charged for activities relating to ISIS since March of 2014, according to a study by the George Washington University’s program on extremism.

At least 10 people were arrested in connection with a foiled plot over the July 4 holiday, in what the government has suggested could have been a major attack during Independence Day.

“There have been numerous arrests of ISIS-inspired people,” said Max Abrahms, a political science professor at Northeastern University who studies terrorism. “But of course for idiosyncratic reasons, there’s going to be variation in perpetrators’ levels of operational success.

“Some attacks will kill more people than others,” he added. This week’s violence in San Bernardino “was just a bloody one — but frankly it could have been a lot worse.”

Federal investigators say that the two suspects at the center of last week’s attack could have been planning a second operation.

White House press secretary Josh Earnest on Friday acknowledged how difficult it is to prevent lone-wolf attacks, calling them “the most significant challenge” facing national security officials.

But Earnest made a sweeping defense of the Obama administration’s counterterrorism policies. He argued that, despite the San Bernardino shooting, law enforcement has a strong track record of foiling lone-wolf plots.

“If you take a look at the track record, and the important work that is done by the FBI and Department of Justice, their record in disrupting these plans before they are carried out is good and should give the American people confidence in the capabilities that we have and the resources that are used to confront this threat,” he said.

A number of factors in this week’s shooting struck analysts as unusual, raising questions for investigators.

For one, the fact that Farook and Malik were husband and wife was curious, since married couples rarely carry out violent rampages.

So was the target: a county health facility where Farook was employed as an environmental health specialist. He reportedly angrily left a holiday party early on Wednesday only to return later heavily armed and with his wife, in a sign that workplace disgruntlement may have also played a role in addition to other motivations.

To onlookers, the violence seemed random and unpredictable.

Yet the White House is rejecting speculation that the government cannot keep Americans safe.

“I don't think the president is resigned to that kind of inevitability,” Earnest said.