ten teenagers. just recently in san bernardino, that killer couple allegedly swore allegiance to isis before gunning down 14 people. authorities worst fear? the couple was hiding in plain sight. as one of the killer's coworkers told our david muir. >> if somebody asked you before yesterday, if he was a friend -- >> i believe so. law enforcement officials are pouring over the recent intelligence, looking for any sign of a threat here at home. and in some cases, some of the suspects of the fbi's radar may have agents knocking on their doors to warn them they're being watched. david? >> pierre thomas live in washington tonight. pierre, thank you. >>> want i want to bring in matt olsen, live with us tonight. and you heard what pierre reported right there. for 24-hour survey illance only some of some of the suspects they're watching around the united states. it could take perhaps dozens of agents. it almost sounds overwhelms. >> well, it is. what we don't have here is the same problem they...