Jack Fleming, of the Boston Athletic Association, which runs the Boston Marathon, pauses at the finish line on Boylston Street between Dartmouth and Exeter Streets, April 22, in Boston. Federal investigators formally released the finish line bombing crime scene to the city of Boston in a brief ceremony at 5 p.m.

Flowers rest at the blast site on Boylston Street between Dartmouth and Exeter Streets near the Boston Marathon finish line, April 22, in Boston.

SWAT teams search for the remaining Boston Marathon bombing suspect in Watertown, Mass., on April 19.

A resident watches as police search an apartment complex in Watertown, Mass., on April 19.

A woman looks out a window at her home as police start to search an apartment building in Watertown on April 19.

Boston Bruins Dennis Seidenberg observes a moment of silence for the victims of the Boston Marathon bombings before the start of an NHL hockey game against the Buffalo Sabres at TD Garden in Boston, Mass., on April 17, 2013. This is the first sporting event to be held in Boston after the explosions that killed three and injured more than one hundred at the Boston Marathon.

A fan wearing a United States flag raises his arms to be checked on the way into TD Garden prior to the game.

Two men in hazardous materials suits put numbers on the shattered glass and debris as they investigate the scene at the first bombing on Boylston Street in Boston on April 16, 2013 near the finish line of the 2013 Boston Marathon.

Boston Marathon bomb scene pictures taken by investigators show the remains of an explosive device. The photos were produced by the Joint Terrorism Task Force of Boston, provided to Reuters April 16, 2013 by a U.S. government official who declined to be identified.

Cousins Conor Gillis, 4, left, and Ben McCormick, 8, take part in a vigil on April 16, in Dorchester, Mass., for the victims of the Boston Marathon bombing.

People gather with candles during a vigil for eight-year-old Martin Richard, from Dorchester, who was killed by an explosion near the finish line of the Boston Marathon on April 16, at Garvey Park in Boston, Massachusetts.

People embrace during a vigil honoring the victims of the Boston Marathon bombings at the Boston Common in Boston, Mass., on April 16.

People gather for a vigil on Boston Common as an investigation continues into dual bombings at the finish line of the Boston Marathon in Boston, Mass., on April 16.

Emma MacDonald, 21, center, cries during a vigil for the victims of the Boston Marathon explosions at Boston Common, April 16.

People move items from a vigil after the barricade they were hanging on was taken down and a portion of Boylston Street reopened a day after two explosions near the finish line of the Boston Marathon in Boston, Mass., on April 16.

A woman places flowers at a barricade which has become a memorial site on Boylston St. at Berkeley St., a few blocks down from the two bomb sites in Boston, Mass., on April 16.

Aaron Jasper, left, and Jackie Pickering hold flowers outside the barricaded entrance at Boylston Street near the finish line of the Boston Marathon in Boston, Mass., on April 16.

The unfinished meals of fleeing customers are left on tables at an outdoor restaurant near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Mass. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence.

Bags of people's belongings gathered not far from the finish line of the Boston Marathon as an investigation continues into dual bombings at the site, in Boston April 16.

Unused thermal blankets for marathon participants are piled near the scene of a twin bombing at the Boston Marathon on April 16, in Boston.

Blood in seen on the sidewalk in front of a candy store advertising a Marathon Monday sale a day after two explosions at the Boston Marathon in Boston, on April 16.

Two police officers walk down Boylston Street, away from the finish line of the Boston Marathon in Boston on April 16.

Runners start the 116th running of the Boston Marathon, in Hopkinton, Mass., Monday, April 16.

Volunteers fill cups at the athlete's village prior to the start of the Boston Marathon, in Hopkinton, Mass., April 16.

Elite women runners, including, Genet Getaneh of Ethiopia, left, Georgina Rono, third from left, Sharon Cherop of Kenya, fourth from left, Firehiwot Dado of Ethiopia, second from right, and Caroline Kilel of Kenya, right, compete in the Boston Marathon in Wellesley, Mass., April 16.