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A morning commuter tears at a make-shift memorial two days after multiple bomb explosions killed three and wounded 176 in Boston, Massachusetts April 17, 2013. Boston Marathon bombing investigators on Wednesday entered the third day of their hunt with an emerging picture of the target: a suspect or suspects carrying heavy bags or backpacks made of dark nylon. While still unable to conclude whether a group or individuals were responsible for the attacks that killed three people and wounded 176, and whether they were foreign or American, investigators gathered enough evidence at the crime scene on Tuesday to slightly narrow their search. REUTERS/Adrees Latif ( USA-EXPLOSIONS/BOSTON )

Investigators search for evidence on the rooftop of a building located above the site of a bomb blast on Boylston Street two-days after multiple explosions at the Boston Marathon killed three and injured 176 in Boston, Massachusetts April 17, 2013. Investigators believe they have identified a suspect in the Boston marathon bombing from security video, a U.S. law enforcement source said. REUTERS/Adrees Latif ( USA-EXPLOSIONS/BOSTON )

Recovery workers, including members of the Federal Bureau of Investigation (FBI), search for clues near the scene of twin bombings at the Boston Marathon on April 17, 2013 in Boston, Massachusetts. The explosions, which occurred near the finish line of the 116-year-old Boston race on April 15, resulted in the deaths of three people with more than 170 others injured. Security has been heightened across the nation as the search continues for the person or people behind the bombings. (Photo by Spencer Platt/Getty Images) ( 166813358 )

Recovery workers, including members of the Federal Bureau of Investigation (FBI), search for clues near the scene of twin bombings at the Boston Marathon on April 17, 2013 in Boston, Massachusetts. The explosions, which occurred near the finish line of the 116-year-old Boston race on April 15, resulted in the deaths of three people with more than 170 others injured. Security has been heightened across the nation as the search continues for the person or people behind the bombings. (Photo by Spencer Platt/Getty Images) ( 166813360 )

Members of the Federal Bureau of Investigation (FBI) search for clues near the scene of twin bombings at the Boston Marathon on April 17, 2013 in Boston, Massachusetts. The explosions, which occurred near the finish line of the 116-year-old Boston race on April 15, resulted in the deaths of three people with more than 170 others injured. Security has been heightened across the nation as the search continues for the person or people behind the bombings. (Photo by Spencer Platt/Getty Images) ( 166813609 )

Boston police officer Pat Duggan writes in chalk outside a makeshift memorial along Newbury street in Boston, Massachusetts April 17, 2013. The investigation of the Boston Marathon bombing is focusing on a suspect or suspects believed to have carried heavy bags or backpacks, but entered a third day on Wednesday without any arrests or word on who was responsible. REUTERS/Shannon Stapleton ( USA-EXPLOSIONS/BOSTON )

Members of the Federal Bureau of Investigation (FBI) search for clues near the scene of twin bombings at the Boston Marathon on April 17, 2013 in Boston, Massachusetts. The explosions, which occurred near the finish line of the 116-year-old Boston race on April 15, resulted in the deaths of three people with more than 170 others injured. Security has been heightened across the nation as the search continues for the person or people behind the bombings. (Photo by Spencer Platt/Getty Images) ( 166813613 )

A member of the Federal Bureau of Investigation (FBI) searches for clues near the scene of twin bombings at the Boston Marathon on April 17, 2013 in Boston, Massachusetts. The explosions, which occurred near the finish line of the 116-year-old Boston race on April 15, resulted in the deaths of three people with more than 170 others injured. Security has been heightened across the nation as the search continues for the person or people behind the bombings. (Photo by Spencer Platt/Getty Images) ( 166813616 )

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. REUTERS ( USA-EXPLOSIONS/BOSTON )

FBI crime scene investigators stand near an evidence marker on Boylston Street just past Berkeley Street near the scene of the Boston Marathon bombing April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166813629 )

An investigator carries an evidence bag off a roof top near the site of two explosions at the finish line of the Boston Marathon in Boston, Massachusetts April 17, 2013. Investigators of the Boston Marathon bombings believe they have identified a suspect from security video taken before Monday's blasts killed three people and injured 176 others, a U.S. law enforcement source said on Wednesday. REUTERS/Brian Snyder ( USA-EXPLOSIONS/BOSTON )

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. REUTERS ( USA-EXPLOSIONS/BOSTON )

An FBI crime scene investigator places an evidence marker on Boylston Street just past Berkeley Street near the scene of the Boston Marathon bombing April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166813632 )

Members of the Federal Bureau of Investigation (FBI) search for clues near the scene of twin bombings at the Boston Marathon on April 17, 2013 in Boston, Massachusetts. The explosions, which occurred near the finish line of the 116-year-old Boston race on April 15, resulted in the deaths of three people with more than 170 others injured. Security has been heightened across the nation as the search continues for the person or people behind the bombings. (Photo by Spencer Platt/Getty Images) ( 166813638 )

A woman in a Boston Marathon runners jacket reaches out to touch a barrier at a makeshift memorial blocks from the finish line on April 17, 2013 in Boston, Massachusetts. Three people were reported killed after two explosions that struck near the finish line of the Boston Marathon on April 15. AFP PHOTO/Stan HONDA/AFP/Getty Images ( 518985165 )

FBI crime scene investigators sweep up Boylston Street after placing an evidence marker down just past Berkeley Street April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166813679 )

FBI crime scene investigators stand near an evidence marker on Boylston Street just past Berkeley Street as they sweep up towards the bomb scene of the Boston Marathon April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166813683 )

Jillian Blenis, 30, of Boston, reacts while stopping at a makeshift memorial, Wednesday, April 17, 2013, in Boston. The city continues to cope following Monday's explosions near the finish line of the Boston Marathon. (AP Photo/Julio Cortez) ( Boston Marathon Explosions )

FBI crime scene investigators sweep up Boylston Street after placing an evidence marker just past Berkeley Street April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166816102 )

FBI crime scene investigators search a truck left on Boylston Street just past Berkeley Street April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166816115 )

FBI crime scene investigators look for evidence just off Boylston Street near Berkeley Street April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166816120 )

FBI crime scene investigators photograph evidence after placing a marker on Boylston Street just past Berkeley Street April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166816128 )

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. REUTERS ( USA-EXPLOSIONS/BOSTON )

FBI crime scene investigators sweep up Boylston Street just past Berkeley Street towards the scene of the bombing looking for evidence April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166816130 )

FBI crime scene investigators sweep up Boylston Street after placing an evidence marker just past Berkeley Street April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166816142 )

An evidence marker placed by FBI crime scene investigators sits on Boylston Street just past Berkeley Street April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166816145 )

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. REUTERS ( USA-EXPLOSIONS/BOSTON )

FBI crime scene investigators look for evidence on Boylston Street after placing an evidence marker just past Berkeley Street April 17, 2013 in Boston, Massachusetts. Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. (Photo by Darren McCollester/Getty Images) ( 166816154 )

An official investigates a rooftop after two explosions hit the Boston Marathon in Boston, Massachusetts April 17, 2013. The investigation of the Boston Marathon bombing is focusing on a suspect or suspects believed to have carried heavy bags or backpacks, but entered a third day on Wednesday without any arrests or word on who was responsible. REUTERS/Shannon Stapleton ( USA-EXPLOSIONS/BOSTON )

Investigators lean over the rooftop of a building located above the site of a bomb blast on Boylston Street two-days after multiple explosions at the Boston Marathon killed three and injured 176 in Boston, Massachusetts April 17, 2013. Investigators believe they have identified a suspect in the Boston marathon bombing from security video, a U.S. law enforcement source said. REUTERS/Adrees Latif ( USA-EXPLOSIONS/BOSTON )

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. REUTERS ( USA-EXPLOSIONS/BOSTON )

Investigators search for evidence on the rooftop of a building located above the site of a bomb blast on Boylston Street two-days after multiple explosions at the Boston Marathon killed three and injured 176 in Boston, Massachusetts April 17, 2013. Investigators believe they have identified a suspect in the Boston marathon bombing from security video, a U.S. law enforcement source said. REUTERS/Adrees Latif ( USA-EXPLOSIONS/BOSTON )

A memorial to Boston Marathon bombing victim, 8-year old Martin Richard is seen near his house in the Dorchester neighborhood of Boston, Wednesday, April 17, 2013. (AP Photo/Michael Dwyer) ( Boston Marathon Explosions )

Investigators look through the top of a building on Boylston Street, two days after two bombs exploded just before the Boston Marathon finish line, Wednesday, April 17, 2013, in Boston. Authorities investigating the deadly bombings have recovered a piece of circuit board that they believe was part of one of the explosive devices, and also found the lid of a pressure cooker that apparently was catapulted onto the roof of a nearby building, an official said Wednesday.(AP Photo/Julio Cortez) ( Boston Marathon Explosions )

A sign lies on the street as investigators comb through one of the blast sites of the Boston Marathon explosions Wednesday, April 17, 2013, in Boston. Authorities investigating the deadly bombings have recovered a piece of circuit board that they believe was part of one of the explosive devices, and also found the lid of a pressure cooker that apparently was catapulted onto the roof of a nearby building, an official said Wednesday. (AP Photo/Julio Cortez) ( Boston Marathon Explosions )

A makeshift memorial to victims, a short distance from the finish line, April 17, 2013 in Boston, Massachusetts, in the aftermath of two explosions that struck near the finish line of the Boston Marathon April 15. AFP PHOTO/Stan HONDA/AFP/Getty Images ( 518985209 )

An FBI investigator, center, walks down a fire truck ladder with a bag from a building at the corner of Boylston Street and Fairfield Street , Wednesday, April 17, 2013, in Boston. Investigators in white jumpsuits fanned out across the streets, rooftops and awnings around the blast site in search of clues on Wednesday. (AP Photo/Julio Cortez) ( Boston Marathon Explosions )

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. REUTERS ( USA-EXPLOSIONS/BOSTON )

Investigators comb through the post finish line area of the Boston Marathon at Boylston Street, two days after two bombs exploded just before the finish line, Wednesday, April 17, 2013, in Boston. (AP Photo/Julio Cortez) ( APTOPIX Boston Marathon Explosions )

Police and investigators scour Boylston Street, near the finish line of Monday's Boston Marathon, in Boston, Wednesday, April 17, 2013. Authorities investigating the deadly bombings at the Boston Marathon have recovered a piece of circuit board that they believe was part of one of the explosive devices, and also found the lid of a pressure cooker that apparently was catapulted onto the roof of a nearby building, an official said Wednesday. (AP Photo/Charles Krupa) ( Boston Marathon Explosions )

An FBI investigator, top, hands a bag to another investigator after walking down a fire truck ladder with it from a building top at the corner of Boylston Street and Fairfield Street , Wednesday, April 17, 2013, in Boston. Investigators in white jumpsuits fanned out across the streets, rooftops and awnings around the blast site in search of clues on Wednesday. (AP Photo/Julio Cortez) ( Boston Marathon Explosions )

FBI investigators walk down a fire truck ladder with bags from a building at the corner of Boylston Street and Fairfield Street , Wednesday, April 17, 2013, in Boston. Investigators in white jumpsuits fanned out across the streets, rooftops and awnings around the blast site in search of clues on Wednesday. (AP Photo/Julio Cortez) ( Boston Marathon Explosions )

Shannon Walsh, 15, places flowers on a memorial to Boston Marathon bombing victim, Martin Richard, 8, near the Richard family house in the Dorchester neighborhood of Boston, Wednesday, April 17, 2013. (AP Photo/Michael Dwyer) ( Boston Marathon Explosions )

An FBI investigator walks down a fire truck ladder with a bag from the top of a building at the corner of Boylston Street and Fairfield Street Wednesday April 17, 2013, in Boston. Investigators in white jumpsuits fanned out across the streets, rooftops and awnings around the blast site in search of clues on Wednesday. (AP Photo/Julio Cortez) ( Boston Marathon Explosions )

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. REUTERS (UNITED STATES - Tags: CRIME LAW DISASTER SPORT ATHLETICS) NO SALES. NO ARCHIVES. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS ( USA-EXPLOSIONS/BOSTON )