A knitted tribute hangs on a street light along the course of the Boston Marathon on the one year anniversary of the 2013 Boston Marathon Bombing, on April 15, in Boston, Mass. Last year, two pressure cooker bombs killed three and injured an estimated 264 others during the Boston marathon, on April 15, 2013. (Andrew Burton/Getty Images) #

Sharon Neary, of Rochester, New York, cries while watching a billboard television screen broadcasting the ceremony commemorating the one year anniversary of the 2013 Boston Marathon Bombing on April 15. Neary said she was standing near the site of the bombing before it went off. (Andrew Burton/Getty Images) #

The Honor Guard marches towards the finish line during the ceremony to remember the Boston Marathon bombing victims. (David L. Ryan/The Boston Globe) #

Ex-mayor Thomas Menino, Boston Mayor Martin Walsh, Vice President Joe Biden, and Governor Deval Patrick observed a moment of silence. (David L. Ryan/The Boston Globe) #

Boston University student, Sebastian Filgueira-Gomez, cries during a moment of silence as he stands on Boylston Street, a block from the finish line. (John Tlumacki/The Boston Globe) #

Ashland, Mass. police officers guard a memorial wreath at the site of the first bomb explosion near the finish line of the Boston Marathon on April 15 in Boston. Vice President Joe Biden attended ceremonies Tuesday to honor victims, first responders, and doctors and nurses who helped them, following the April 15, 2013 bombings, during a tribute in Boston. (Steven Senne/Associated Press) #

2013 Boston Marathon bombing survivor Erika Brannock, a pre-school teacher from the Baltimore area, and her mother, Carol Downing, at right, walk across the Marathon finish line after a remembrance ceremony on Boylston Street in Boston on April 15. (Elise Amendola/Associated Press) #

Boston Police officer James Kenneally gave Joe Jackson of Dorchester, Mass., a high five as he watched the parade of people who came from the ceremony at the Hynes Auditorium on their way to the flag ceremony at the finish line. (Suzanne Kreiter/The Boston Globe) #

A Boston Police officer stands guard at the finish line of the Boston Marathon prior to the flag raising ceremony commemorating the one-year anniversary of the Boston Marathon bombings on Boylston Street near the finish line on April 15. (Jared Wickerham/Getty Images) #

A law enforcement official searches a man near the finish line of the Boston Marathon on April 15 in Boston. Vice President Joe Biden was among the dignitaries Tuesday during ceremonies to honor the victims, first responders, doctors and nurses who helped them, after the Boston Marathon bombings, at the Hynes Convention Center in Boston. (Steven Senne/Associated Press) #

Pedestrians pass by blue and yellow banners near the site of one of the two bomb blasts on the one-year anniversary of the 2013 Boston Marathon bombings in Boston on April 15. (Dominick Reuter/Reuters) #

Kevin Brown puts up a hand made memorial for victims of the 2013 Boston Marathon bombings near the race's finish line in Boston on April 15, the one-year anniversary of the bombings. (Brian Snyder/Reuters) #

Olivia Savarino, center, hugs Christelle Pierre-Louis, left, as Callie Benjamin, right, looks on near the finish line of the Boston Marathon during ceremonies on Boylston Street. Savarino and Benjamin were working at the Forum restaurant when a bomb went off in front of the building on April 15, 2013. (Steven Senne/Associated Press) #

People photograph a banner reading "Boston Strong" hanging at Rowes Wharf on the first anniversary of the Boston Marathon bombings on April 15. (Bill Sikes/Assocaited Press) #

Tim Sorbanelli, 21, a Berklee College of Music student, plays his guitar near the finish line of the Boston Marathon prior to the flag raising ceremony commemorating the one-year anniversary of the Boston Marathon bombings on Boylston Street near the finish line on April 15. (Jared Wickerham/Getty Images) #

People watch a billboard television screen broadcasting the ceremony commemorating the one-year anniversary of the 2013 Boston Marathon bombings. Last year, two pressure cooker bombs killed three and injured an estimated 264 others during the Boston marathon, on April 15, 2013. (Andrew Burton/Getty Images) #

A running shoe with a bouquet of flowers is left beside the barricades in front of Marathon Sports where the first bomb went off near the Boston Marathon finish line on the one-year anniversary of the bombings. (Jessica Rinaldi/The Boston Globe) #

Mayor Martin J. Walsh,right, expressed condolences to the family of bombing victim Martin Richard including his parents Denise, Bill, Jane (sister) and Henry (brother) during a wreath laying ceremony commemorating the one-year anniversary of the Boston Marathon bombings. (Wendy Maeda/The Boston Globe) #

The shoes of 2013 Boston Marathon bombing survivor J.P. Norden read "Boston Strong" as he stands at the finish line on the one-year anniversary of the bombings in Boston, Mass., on April 15. J.P. and his brother Paul, also a bombing survivor, took part in the final portion of the "Legs for Life Relay", joining family members and friends who walked the entire Marathon route to raise money for children needing prosthetic limbs. US Vice President Joe Biden, other leaders and survivors of the Boston Marathon bombing shared messages of thanks and defiance on Tuesday at a tribute to the three people killed and 264 wounded in the attack exactly one year ago. (Brian Snyder/Reuters) #