This photo provided by the San Bernardino County Sheriff's Department shows four guns in a shooting that the FBI is investigating as a terrorist attack. (Handout/Getty Images)

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, 27, 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.

FBI Director James Comey said at a news conference on Dec. 4 that the investigation into attackers who shot and killed 14 people in California is now being investigated as an act of terrorism. (AP)

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.

“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 were 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 killed130 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.”

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 that 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.

Michael G. Vickers, who served as the top intelligence official at the Pentagon until earlier this year, said that the San Bernardino attacks could push the FBI to consider lower thresholds for opening terrorism-related inquiries, perhaps even doing so for seemingly innocuous online links.

“Would you lower the bar to someone liking someone on Facebook?” Vickers said, citing disclosures that Farook may have had such links to others with extremist views. “Would you give them a little attention rather than no attention?”

Doing so would probably require a major shift in resources by the FBI, which divides its massive workforce about equally between a criminal division that pursues traditional cases­ and the national security branch that expanded dramatically after the Sept. 11, 2001, attacks.

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.

The FBI relies heavily on sting operations to bait would-be terrorists and has created a network of task forces across the country to improve collaboration among law enforcement agencies and to identify sites potentially vulnerable to attack.

The bureau helped disrupt a plot with ties to the Islamic State earlier this year by pushing local police to increase security at an event in Garland, Tex., that seemed designed to provoke Muslims by inviting participants to draw cartoons mocking the prophet Muhammad.

Two Phoenix men, including one who had been under FBI investigation, tried to open fire on the contest, but both were killed before any participants could be targeted. One of the gunmen had exchanged hundreds of messages with an Islamic State operative overseas, but Comey said months later that the bureau still didn’t know the contents of those messages because they were encrypted.

The ability to detect a plot is diminished further when there is little or no communication to intercept.

FBI officials said Friday that they are investigating Farook’s “telephonic conversations” with at least one other individual in the United States. But the most meaningful clues to surface so far have to do with the weapons, uniforms and explosives the couple assembled — as well as their preparations on how to use them — through a series of transactions that drew little or no attention from authorities.

The weapons included semi­automatic rifles whose configurations suggested a familiarity with military doctrine. Their grips and straps were set up identically so that each could be operated by either attacker, enabling each to pick up the other’s weapon and resume firing seamlessly if the other were injured or killed.

The abundant preparations for the San Bernardino assault, and the involvement of a woman, echoed aspects of the attacks by teams of militants in Paris. But the violence in France exploited vulnerabilities in European security that, so far, outstrip the threat in the United States.

Several of the Paris plotters had traveled to Syria. The architect of the attack had even appeared in Islamic State recruitment videos. They were among roughly 4,500 European citizens suspected of traveling to Syria since the start of that country’s civil war, compared with just 250 who have traveled there or attempted to from the United States.