A Russian warplane taxies at Hemeimeem airbase in Syria. Advocates say a no-fly zone would keep Syrian aircraft from attacking anti-government rebels and endangering civilians, which might allow Syrians to feel safe enough to stay put. Vladimir Isachenkov/AP hide caption

toggle caption Vladimir Isachenkov/AP

A biologist holds a slide prepared for testing in a micro array for biological hazards at the Lawrence Livermore National Laboratory in Livermore, Calif. Ben Margot/AP hide caption

toggle caption Ben Margot/AP