"If you see something, say something" is the slogan in the fight against terrorism in the United States.
It is a call for citizens to immediately call police if they see anything out of the ordinary in their daily surroundings, no matter how innocent it may appear — an abandoned suitcase, an out-of-place box or a bottle of unfamiliar liquid.
FBI chief James Comey said he knew how unsettling incidents like Wednesday's California massacre could be. He urged Americans on Friday not to become distracted and disabled by fear.
"If you see something that doesn't make sense, you say something to somebody," he said. "When we look back over our cases over the last 10 to 15 years, in almost any case, we find that somebody saw something — whether it was a family member or friend or a co-worker — and didn't say something to law enforcement."
But one expert on terrorism and extremism said the California killings gave Americans reason to be extra concerned.
Both suspects, Syed Farook and his wife, Tashfeen Malik, appeared to be a rather ordinary couple, living quietly as they raised their 6-month-old daughter.
There have been no reports that either showed any aggressive or pro-extremist behavior, or talked openly about terrorism or the Islamic State group, or raised any suspicions among family, friends or co-workers.
Comey said there was nothing of significance that put the killers on what he called the FBI's "radar screen."
Thomas Sanderson, director of transnational threats at the Center for Strategic and International Studies, said Farook was "a guy who grew up in America. He may have thought that with a wife and a child he may not attract that kind of attention, and I think that's a key thing to consider here.
"I think for the husband, if he is the one who led this, he would very much see a trusted wife who he recently married, probably married specifically to have someone provide backup for him ... to be a second set of eyes and ears as you prepare for and execute the attack."
Sanderson said the couple could be a role model for U.S. adherents to Islamic State.
"It's all theoretical, attacking inside the United States, until someone does it. And then, boom. You've got your sample, and essentially it's a proxy force for ISIS," he said, using an acronym for Islamic State. "It strikes fear in the hearts of Americans ... because this was a strike at an office and not a strike on a financial center or a strike on the Pentagon or the FBI headquarters."