Boston Marathon

Holly Holland, right, hugs her daughter Katie, center, as her husband Kevin Holland, left, all of St. Louis,  looks while visiting a makeshift memorial in Boston, Monday, April 22, 2013. The memorial sits on Boylston St., not far from where two bombs exploded near the finish line of the Boston Marathon, Monday, April 15, 2013

Steven Senne/AP

Holly Holland hugs her daughter Katie, both of St. Louis, while visiting a makeshift memorial in Boston, on Monday. The memorial sits on Boylston Street, not far from where two bombs exploded near the finish line of the Boston Marathon, last week.

WATERTOWN, MA - APRIL 19:  Police with guns drawn search for a suspect on April 19, 2013 in Watertown, Massachusetts. Earlier, a Massachusetts Institute of Technology campus police officer was shot and killed late Thursday night 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.

Mario Tama/Getty

Police with guns drawn on Friday search for a suspect in Watertown, Massachusetts. A Massachusetts Institute of Technology campus police officer was shot and killed late on Thursday night at the school’s campus in Cambridge. A short time later, police reported exchanging gunfire with alleged carjackers in Watertown, a city near Cambridge. 

A jetliner flies over flags flying at half-staff at the Washington Monument in Washington, Tuesday, April 16, 2013

David Ake/AP

A jetliner flies over flags flying at half-staff at the Washington Monument in Washington, Tuesday, April 16, 2013, after President Obama ordered flags to be lowered on federal building to honor the loss of life from the explosions at the Boston Marathon.