According to White House tumblr, Google co-founder Sergey Brin's family were refugees from the Soviet Union, coming to the US when Brin was 6. Researching further,
Since 1975 more than 500,000 refugees, large numbers of whom were Jews, evangelical Christians, and Catholics from the former Soviet Union, have been resettled in the United States. (wikipedia)
How did the US ensure that no infiltrators, saboteurs or other security threats blended in with this large number of refugees from a hostile country? How successful were they?