Israeli police inspectors stand next to a bus damaged in an explosion March 23, 2011 in Jersalem, Israel. A bomb exploded near a crowded bus, wounding passengers in what appeared to be the first militant attack in the city in several years.

An Iraqi woman grieves at the scene of a bomb attack in Jameela market in the Iraqi capital's crowded Sadr City neighborhood Baghdad, Iraq, Thursday, August 13, 2015. A massive truck bomb ripped through a popular Baghdad food market in a predominantly Shiite neighborhood in the early morning hours on Thursday, killing dozens of people, police officials said. (Photo by Karim Kadim/AP Photo)

This combination of April 15, 2013 and April 2, 2014 photos show a police officer clearing Boylston Street following a bomb blast near the finish line of the 2013 Boston Marathon, and pedestrians crossing at the same corner nearly a year later in Boston. (Photo by Charles Krupa/AP Photo)

Women comfort each other as they mourn over the death of a family member who was killed in a bomb blast, at a local hospital in Lahore, Pakistan, Sunday, March, 27, 2016. A bomb blast in a park in the eastern Pakistani city of Lahore has killed tens of people and wounded scores, a health official said. (Photo by K.M. Chuadary/AP Photo)

A North Korean soldier throws a stone towards a photographer on the banks of the Yalu River in the North Korean town of Sinuiju, opposite the Chinese border city of Dandong October 21, 2006 in Sinuiju, Democratic People's Republic of Korea. (Photo by Cancan Chu/Getty Images)

Campers set structures on fire in preparation of the Army Corp's 2pm deadline to leave the Oceti Sakowin protest camp on February 22, 2017 in Cannon Ball, North Dakota. Activists and protesters have occupied the Standing Rock Sioux reservation for months in opposotion to the completion of the Dakota Access Pipeline. (Photo by Stephen Yang/Getty Images)

In this Thursday, December 1, 2016 photo, Cat Bigney, part of the Oglala Native American tribe, waits on the shore of the Cannonball river for travelers to arrive by canoe at the Oceti Sakowin camp where people have gathered to protest the Dakota Access oil pipeline in Cannon Ball, N.D. So far, those at the camp have shrugged off the heavy snow, icy winds and frigid temperatures. But if they defy next week's government deadline to abandon the camp, demonstrators know the real deep freeze lies ahead. Life-threatening wind chills and towering snow drifts could mean the greatest challenge is simple survival. (Photo by David Goldman/AP Photo)

An African immigrant tries to jump a fence into a ferry terminal in the western Greek town of Patras April 28, 2015. Afghan, Iranian and Sudanese immigrants, living precariously in abandoned factories in Patras, southwest Greece, try to stow away on nearby ferries to Italy as they seek a better life in Europe beyond crisis-hit Greece. (Photo by Yannis Behrakis/Reuters)