The San Bernardino attack that left 14 people dead represented a type of extremist plot law enforcement authorities consider exceedingly difficult to detect: a conspiracy between close family members.
While investigators are still trying to piece together the details of the plot, husband and wife Syed Rizwan Farook and Tashfeen Malik appear to have dropped precious few clues to what they were planning.
"They're not even below the radar. They're stealth," said Patrick Skinner, a former CIA case officer who is now a security consultant. "Basically you have a woman with zero friends, who had no contact with anyone, and a husband who was the same way. That's a tough nut to crack."
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A small group of plotters is, by its very nature, harder to detect. The more members a terror cell has, the greater the chances someone will slip up and expose the plan to someone on the outside.
But detecting, infiltrating and thwarting small groups of terrorists is exceptionally tough when the brothers in arms are, in fact, brothers — or father and son, or husband and wife, security experts say.
Family members don't need computers or phones to communicate and can conspire around the kitchen table or in bed, beyond the reach of surveillance. Kin may be more reluctant to betray one another. And law enforcement, even if it catches wind of a conspiracy, may be powerless to insert an informant or an undercover agent into such a tight circle.
"If you have a cell that's all related, they have either blood relations or they're related through marriage, you're less likely to find a weakness in that group," said Ed Davis, who was police commissioner in Boston in 2013 when brothers Dzhokhar and Tamerlan Tsarnaev bombed the Boston Marathon, killing three people and wounding 260.
Davis has seen the same phenomenon in Boston's underworld, where mobsters and bank robbery rings with deep personal or family ties adhere to a code of silence.
For centuries, blood, marriage and romance have bound criminals, including Old West outlaws Jesse James and his brother Frank, the Depression-era bank robbers Bonnie and Clyde, and the Mafia.
It was a pair of brothers, Said and Cherif Kouachi, who carried out the attacks in Paris in January on the satirical newspaper Charlie Hebdo.
And multiple sets of brothers figured in the latest Paris attacks, among them Salah and Brahim Abdeslam. Brahim died in a suicide bombing, while Salah, who handled logistics for the attacks, is a fugitive. The suspected mastermind, Abdelhamid Abaaoud, enrolled his brother as a fighter for Islamic State. After Abaaoud himself was killed in a police raid, his brother threatened revenge.
Several thwarted attacks in the U.S. have involved family members.
Two Illinois cousins pleaded guilty this month to conspiring in a pair of plots that would have had one man storming the Joliet armory and the other jetting off to fight overseas with the Islamic State. Three brothers originally from Macedonia were among five men convicted of conspiring to kill military personnel at Fort Dix in New Jersey in 2007. A father and two sons from North Carolina pleaded guilty in 2011 to plotting jihadist attacks against the American military.
Most of the jihadist conspiracies uncovered in the U.S. involve someone who thinks he is reaching out to an extremist group such as al-Qaida or the Islamic State but is actually dealing with someone inserted by the FBI, said Brian Michael Jenkins, a terrorism expert at the RAND Corp.
In investigating the Dec. 2 massacre in San Bernardino, the FBI said it discovered that Farook and Malik communicated online about jihad and martyrdom before Malik immigrated to the U.S. But they did it through direct, private messages, not postings on social media, according to the FBI.
The FBI is still investigating whether they worked directly with any terror organizations. Around the time they launched the attack on Farook's colleagues from the county health department, they declared allegiance online to the Islamic State.
Farook did, in fact, reach outside his marriage for help in at least one instance: A friend and former next-door neighbor, Enrique Marquez Jr., supplied the assault rifles used in the rampage, and he and Farook plotted attacks in 2011 and 2012 that were never carried out, investigators said Thursday.
But even there, there was a family connection: Marquez became related to Farook last year by entering into what prosecutors said was a sham marriage to help a Russian woman obtain U.S. residency.
Still, contact between Farook and Marquez began to decline in 2013, according to court papers. Friends, co-workers and family lawyers have said Farook and his wife were extremely isolated. And nothing has come to light about what the two discussed in the days leading up to the attack.
"We would have to be sitting at the kitchen table for that," Skinner said.