Nebraska and Oklahoma say Colorado's marijuana law is unconstitutional, in a challenge to the law in the Supreme Court. Earlier this month, visitors from Texas smell marijuana at the Breckenridge Cannabis Club. Brennan Linsley/AP hide caption

itoggle caption Brennan Linsley/AP

Health workers in protective suits transport Dr. Martin Salia, a surgeon working in Sierra Leone who has been diagnosed with Ebola, from a jet that brought him from Sierra Leone to a waiting ambulance in Omaha, Neb., on Saturday. Nati Harnik/AP hide caption

itoggle caption Nati Harnik/AP

A still image from a video shows two tornadoes approaching Pilger, Neb., Monday. The National Weather Service said at least two people died in the large storm that hit northeast Nebraska. StormChasingVideo.com/AP hide caption

itoggle caption StormChasingVideo.com/AP

A TransCanada Keystone Pipeline pump station operates outside Steele City, Neb. The State Department is extending the review period for the pipeline, given ongoing litigation in Nebraska over the project. Lane Hickenbottom/Reuters /Landov hide caption

itoggle caption Lane Hickenbottom/Reuters /Landov