martha raddatz is in brussels >> reporter: tonight in buss bresz els, in the heart of the city, the police and military blocking off the streets. briskly moving pedestrians along. and warning journalists after a threat was reported. >> if we start shooting you are in the crossfire. >> reporter: it appeared to be a false alarm, one of several tonight. but it is no wonder the poce are on edge, again tomorrow this city, wi be on lockdown, the subways shuttered, schools closed, shopping malls, restaurants, all potential targets in what belgian officials fear is an imminent mass casualty attack, by multiple a assailantin multie locations. the paris scenario. the u.s. embassy urging american citizens to shelter in place. this as the manhunt for believed to be on this surveillance video. authorities say he drove to belgium, passing unnoticed through three security checks. the lawyer for his companions telling abc news he may have been wearing a suicide belt. abdeslam's brother saying in interviews today that the family though he was going on a ski trip when he disappeared prior to the attacks, an