Unless there is probable cause to believe that someone has done something wrong, I don't know how you say that the security of anyone is threatened by that individual. If there is probable cause, then the government can get a warrant.If they don't have probable cause, what do they have? Improbable cause?
Not "has done something wrong". After the fact doesn't matter. If the crime has been committed then it cannot be undone and the needed time for a court order is no problem. These intercepts are supposed to gain info on and from people who are definitely connected to terrorists and are intended to PREVENT an attack. The president and AG have said it is a time factor and the intercept needs to be done NOW because the call is being made NOW. Time is of the essence and again it has been made clear that the target in every instance is known to have a connection to terrorism.