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