u.s. authorities investigate threaten letter send republican presidential candidate donald trump sister federal appeal court judge philadelphia accord nbc news maryanne trump barry 78 receive letter friday network philadelphia affiliate say report late sunday cite source familiar investigation reuters could immediately confirm report separately friday officials say investigate letter send trump son eric contain granular substance fbi u.s. secret service investigate letter trump sister source tell nbc letter contain white powder threaten message demand trump drop bid republican presidential nominee trump lead among three remain republicans campaign represent party nov. presidential election fbi secret service probe letter send trump 's sister nbc