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All in the family: Terrorists connected by blood or marriage can be more difficult to root out

Associated Press
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FILE - This July 27, 2014 file photo provided by U.S. Customs and Border Protection shows Tashfeen Malik, left, and Syed Farook, as they passed through O'Hare International Airport in Chicago. The shooters suspected in the San Bernardino massacre operated under a cover rarely used to plot terror. Marriage provided Farook and Malik a shield that would have been nearly impossible for law enforcement to penetrate, even if they knew the couple had become radicalized. (U.S. Customs and Border Protection via AP, File) (The Associated Press)

FILE - This combination of file photos shows Tamerlan, left, and Dzhokhar Tsarnaev, brothers who planted bombs at the finish line at the Boston Marathon on April 15, 2013. Detecting and stopping small groups of terrorists is especially difficult when the brothers in arms are, in fact, brothers or cousins, fathers and sons or even wives. Family members can scheme in person in a kind of kitchen table conspiracy beyond the reach of surveillance. Kin may be reluctant to betray any suspicions. And law enforcement, if they catch wind of a conspiracy, may be powerless to plant a mole in such a tight circle. (AP Photos/Lowell Sun and FBI, File) (The Associated Press)

The San Bernardino terror attack represented a type of extremist plot authorities consider exceedingly difficult to detect: a conspiracy between close family members.

The husband and wife who killed 14 people appear to have dropped precious few clues to what they were planning.

Security experts say a small group of plotters is 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.

Several major terrorist plots have involved family members, including attacks in Paris and the Boston Marathon bombing.