WATERTOWN, MA - APRIL 19: Police descend on School and Walnut Street chasing possible suspects on April 19, 2013 in Watertown, Massachusetts. Earlier, a Massachusetts Institute of Technology campus police officer was shot and killed at the school's campus in Cambridge. A short time later, police reported exchanging gunfire with alleged carjackers in Watertown, a city near Cambridge. It's not clear whether the shootings are related or whether either are related to the Boston Marathon bombing. (Photo by Darren McCollester/Getty Images)