The FBI has nearly 1,000 open cases related to violent extremism touching every U.S. state. The bureau is tracking hundreds of people with suspected ties to terrorist groups and has charged at least 60 of them with crimes connected to the Islamic State. Yet no strand of that net ever settled near the shooters in San Bernardino, Calif. Their paramilitary-style assault combined aspects of the recent attacks in Paris with the steady stream of U.S. gun violence in a way that authorities seemed powerless to prevent.

The emerging chronology has presented some unnerving clues. Syed Rizwan Farook, 28, and his wife, Tashfeen Malik, 29, assembled a small arsenal of weapons in their home while developing an allegiance to the Islamic State that authorities said was revealed in a declaration on Facebook.

U.S. officials have also said that one or both of the attackers had previously been in contact with individuals whose extremists views came under scrutiny from the FBI, although those contacts were described as so innocuous that the bureau saw no reason to probe further.

U.S. officials and counterterrorism experts said the hybrid nature of the attack - mixing an Islamist agenda, possible workplace grievance, and legally acquired weaponry - exposed a threat matrix that may strain domestic security agencies' capabilities, no matter how aggressively they seek to adapt.

New challenge

"Given the target and given the strangeness of this whole attack, you have to wonder whether there isn't some fusion here which will be a challenge for us," said Daniel Benjamin, a professor at Dartmouth College who previously served as a senior counterterrorism official in the Obama administration. "The motives are so mixed that [those involved] don't really show up on any screen when it comes to radical ideology or affiliation."

Despite scouring laptops, cellphones, and the online accounts of the shooters for two days, U.S. officials said Friday that they had yet to unearth any evidence that Farook or Malik was directed by operatives from the Islamic State.

Nor did that terrorist group rush to claim credit for the San Bernardino violence the way it did with bombastic propaganda releases after the attacks in Paris last month killed 130 people. The absence of such claims has led some to conclude that the Islamic State was as surprised by the San Bernardino shooting as authorities in the United States.

The insulated nature of the plot is a scenario that U.S. counterterrorism officials describe with dread.

"This is something we've talked about a lot," FBI Director James B. Comey said Friday. "Because the Internet offers the ability for people to consume poison and radicalize entirely in private either through a device they are holding in their hand or inside their house, our visibility is necessarily limited. And so we constantly worry . . . about who is out there on this journey from consuming poison to acting on it and can we get eyes on them in time to stop it."

Shift to online

The attack makes clear how completely the terrorist threat has shifted online. A decade ago, the couple's trips to Saudi Arabia and links to Pakistan would have triggered scrutiny of their travels, searching for clues that they had become radicalized through direct contact with al-Qaeda operatives. Instead, FBI agents are now scrubbing hard drives for encrypted messages and searching for motives in a Facebook post.

Current and former U.S. officials said a determination that the San Bernardino attack was inspired by the Islamic State could put pressure on the Obama administration to intensify its military campaign against the group and to expand the criteria that triggers domestic scrutiny of potential terrorist suspects.

The FBI shifted hundreds of agents from the criminal division to deal with the Islamic State threat earlier this year, amid growing concern over the number of Americans being recruited or inspired by the terrorist group.

Those agents were returned to the criminal division as the number of Americans seeking to travel to Syria tapered off. But the attacks in Paris and San Bernardino have raised new concerns about the group's reach and could force Comey to reconsider how bureau personnel are deployed.

INSIDE

The president of Liberty University urges more students to carry guns. A4.

Clinics step up security after Colo. attack. A5.

The New York Times calls for greater gun control in a front-page editorial. A14.

We must mute the siren song of ISIS. Editorial, C4.

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