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.