Seriously than initially known. the u.s. state department says at least 16 americans have suffered symptoms and some have suffered brain injuries. some of the attacks were inaudible, others made a deafening sound described as the buzzing of an insect swarm or metal scraping across a floor. does the u.s. know precisely who is behind this? >> not at this point. multiple u.s. agencies are still investigating. we're told that the cubans have been responsive, but they vehemently deny having anything to do with this. the u.s. isn't ruling anything out. and the details, some of which you mentioned ar...