By Darren McCollester

BOSTON, MA - APRIL 15: Swat teams arrive at the corner of Stuart Street and Dartmouth Street after two explosive devices detonated at the finish line of the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.

By Darren McCollester 
BOSTON, MA - APRIL 16: Swat team members and special police units assemble in the Boston Common on April 16, 2013 in Boston, Massachusetts. Security is especially tight in the city of Boston after two explosions went off near the finish of the Marathon, killing three people and injuring at least 141 others.

BOSTON, MA - APRIL 16: A police officer stands guard at the corner of Boylston and Arlington Streets with the marathon finish line bridge in the distance on April 16, 2013 in Boston, Massachusetts. Security is especially tight in the city of Boston after two explosions went off near the finish of the Marathon, killing three people and injuring at least 141 others. (Photo by Darren McCollester

April 15, 2013; Boston, MA, USA; An FBI officer walks down the street in front of the Westin Hotel after several explosions occurred at the Boston Marathon. (Bob DeChiara-USA TODAY Sports)

April 15, 2013; Boston, MA, USA; A Boston ambulance is parked near a medical tent after several explosions occurred earlier in the day at the Boston Marathon. (Bob DeChiara-USA TODAY Sports)

April 15, 2013; Boston, MA, USA; Military medics stand guard outside the Westin Hotel after several explosions occurred earlier in the day at the Boston Marathon. (Bob DeChiara-USA TODAY Sports)

April 15, 2013; Boston, MA, USA; Military personnel stand guard at the entrance of the Sheraton hotel after several explosions occurred earlier at the Boston Marathon. (Bob DeChiara-USA TODAY Sports)

By Alex Trautwig 

BOSTON, MA - APRIL 15: The outter perimeter near Massachusetts Avenue and Newbury Street is secured by police after two explosive devices detonated at the finish line of the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. 

Apr 15, 2013; Boston, MA, USA; Boston police patrol following multiple bombings near the finish line of the 2013 Boston Marathon. (Michael Ivins-USA TODAY)

By Alex Trautwig

BOSTON, MA - APRIL 15: State police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon.

By Darren McCollester 

BOSTON, MA - APRIL 16: Police units assemble along the Boston Public Garden on April 16, 2013 in Boston, Massachusetts. Security is especially tight in the city of Boston after two explosions went off near the finish of the Marathon, killing three people and injuring at least 141 others.