Boston Marathon

BOSTON_11337

** FILE ** A firefighter tends to an injured man following an explosion near the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Michael Dwyer)

Boston Marathon-Explo_Lanc.jpg

An unidentified Boston Marathon runner is comforted as she cries in the aftermath of two blasts which exploded near the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Elise Amendola)

SECURITY_11335

A U.S. Parks Police officer with an assault rifle patrols the Washington Monument in Washington, DC., after a bombing at the Boston Marathon in Boston, Mass., April 15, 2013. (Andrew S Geraci/The Washington Times)

SECURITY_11334

Police have moved visitors to the north lawn of the White House back across the street as a precaution after bombs explode at the finish line of the Boston Marathon, Washington, D.C., Monday, April 15, 2013. (Andrew Harnik/The Washington Times)

SECURITY_11333

U.S. Parks Police with assault rifles patrol the Washington Monument in Washington, DC., after a bombing at the Boston Marathon in Boston, Mass., April 15, 2013. (Andrew S Geraci/The Washington Times)

SECURITY_11332

Police block off Pennsylvania Ave., from pedestrian traffic in front of the White House as a precaution after bombs explode at the finish line of the Boston Marathon, Washington, D.C., Monday, April 15, 2013. (Andrew Harnik/The Washington Times)

SECURITY_11329

Students from Bolder, Colo. stand behind police tape after police block off Pennsylvania Ave., from pedestrian traffic in front of the White House as a precaution after bombs explode at the finish line of the Boston Marathon, Washington, D.C., Monday, April 15, 2013. (Andrew Harnik/The Washington Times)

Obama_Boston_Marathon_Exp#2.JPG

President Barack Obama speaks in the James Brady Press Briefing Room at the White House in Washington, Monday, April 15, 2013, following the explosions at the Boston Marathon. (AP Photo/Charles Dharapak)

Obama with Mueller.jpg

President Obama talks on the phone with FBI Director Robert Mueller following explosions at the Boston Marathon. (Pete Souza/White House photo)

Boston Marathon Aerial View.jpg

Aerial view of the aftermath of the explosions near the finish line of the Boston Marathon. (Twitter phot: Bruce Mendelsohn)

BOSTON_11312

Medical personnel work outside the medical tent in the aftermath of two blasts which exploded near the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Elise Amendola)

Tyler Wakstein photo.jpg

Photo of the aftermath of a suspected explosion at the finish line of the Boston Marathon. (Twitter photo/Tyler Wakstein)

Boston Marathon_Lanc(4).jpg

Rita Jeptoo, of Kenya, crosses the finish line to win the women's division of the 2013 running of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Charles Krupa)

Boston Marathon_Lanc(2).jpg

Elite mens marathoners including Levy Matebo, left, and Markos Geneti, right, run in the 117th Boston Marathon in Wellesley, Mass., Monday, April 15, 2013. (AP Photo/Michael Dwyer)