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Pallbearers carry the casket of fallen Massachusetts Institute of Technology police officer Sean Collier into St. Patrick's Church before a funeral Mass, in Stoneham, Mass., Tuesday, April 23, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Steven Senne) ( Boston Marathon Officer Funeral )

Mourners enter St. Patrick's Church in Stoneham, Mass., before a funeral Mass for Massachusetts Institute of Technology police officer Sean Collier, Tuesday, April 23, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Steven Senne) ( Boston Marathon Officer Funeral )

Richard Corapi holds a U.S. flag from a sidewalk as Massachusetts Institute of Technology police officers march away from the funeral of fellow MIT police officer Sean Collier in Stoneham, Massachusetts April 23, 2013. Officer Collier was allegedly killed by the brothers accused of the Boston Marathon bombings, Dzhokhar and Tamerlan Tsarnaev. Two explosions hit the Boston Marathon April 15 killing at least three people and injuring over 100 others. REUTERS/Brian Snyder ( USA-EXPLOSIONS/BOSTON )

Police officers march in formation as they enter St. Patrick's Church, in Stoneham, Mass., before a funeral Mass for Massachusetts Institute of Technology police officer Sean Collier Tuesday, April 23, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Steven Senne) ( Boston Marathon Bombings )

Pallbearers carry the casket of slain Massachusetts Institute of Technology (MIT) police officer Sean Collier into the hearse following the funeral at St Patrick's Church on April 23, 2013 in Stoneham, Massachusetts. Collier was allegedly shot by the suspects in the Boston Marathon bombing, Dzhokhar A. Tsarnaev, 19, and his brother Tamerlan Tsarnaev, 26, after their identities were determined and a manhunt was launched. Tamerlan Tsarnaev was subsequently shot and killed after a car chase and shootout with police and Dzhokhar Tsarnaev was apprehended on a boat parked on a residential property in Watertown, Massachusetts. The bombing, on April 15 at the finish line of the marathon, killed three people and wounded at least 170. (Photo by Jared Wickerham/Getty Images) ( 167266874 )

A Somerville Police officer salutes as he walks past the hearse carrying the casket of the slain Massachusetts Institute of Technology (MIT) police officer Sean Collier following his funeral at St Patrick's Church on April 23, 2013 in Stoneham, Massachusetts. Collier was allegedly shot by the suspects in the Boston Marathon bombing, Dzhokhar A. Tsarnaev, 19, and his brother Tamerlan Tsarnaev, 26, after their identities were determined and a manhunt was launched. Tamerlan Tsarnaev was subsequently shot and killed after a car chase and shootout with police and Dzhokhar Tsarnaev was apprehended on a boat parked on a residential property in Watertown, Massachusetts. The bombing, on April 15 at the finish line of the marathon, killed three people and wounded at least 170. (Photo by Jared Wickerham/Getty Images) ( 167266888 )

Law enforcement officials depart St. Patrick's Church in Stoneham, Mass., following a funeral Mass for Massachusetts Institute of Technology police officer Sean Collier, Tuesday, April 23, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Steven Senne) ( Boston Marathon Officer Funeral )

Boston Archbishop Cardinal Sean O'Malley, center, departs St. Patrick's Church in Stoneham, Mass., with other members of the clergy following a funeral Mass for Massachusetts Institute of Technology police officer Sean Collier, Tuesday, April 23, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Steven Senne) ( Boston Marathon Officer Funeral )

A priest greets family and friends of Massachusetts Institute of Technology (MIT) police officer Sean Collier after his funeral in Stoneham, Massachusetts April 23, 2013. Officer Collier was allegedly killed by the brothers accused of the Boston Marathon bombings, Dzhokhar and Tamerlan Tsarnaev. Two explosions hit the Boston Marathon April 15 killing at least three people and injuring over 100 others. REUTERS/Brian Snyder ( USA-EXPLOSIONS/BOSTON )

Mourners embrace one another outside of St Patrick's Church following the funeral for the slain Massachusetts Institute of Technology (MIT) police officer Sean Collier on April 23, 2013 in Stoneham, Massachusetts. Collier was allegedly shot by the suspects in the Boston Marathon bombing, Dzhokhar A. Tsarnaev, 19, and his brother Tamerlan Tsarnaev, 26, after their identities were determined and a manhunt was launched. Tamerlan Tsarnaev was subsequently shot and killed after a car chase and shootout with police and Dzhokhar Tsarnaev was apprehended on a boat parked on a residential property in Watertown, Massachusetts. The bombing, on April 15 at the finish line of the marathon, killed three people and wounded at least 170. (Photo by Jared Wickerham/Getty Images) ( 167266881 )

Members of many different law enforcement agencies walk in formation into the funeral for the slain Massachusetts Institute of Technology (MIT) police officer Sean Collier at St Patrick's Church on April 23, 2013 in Stoneham, Massachusetts. Collier was allegedly shot by the suspects in the Boston Marathon bombing, Dzhokhar A. Tsarnaev, 19, and his brother Tamerlan Tsarnaev, 26, after their identities were determined and a manhunt was launched. Tamerlan Tsarnaev was subsequently shot and killed after a car chase and shootout with police and Dzhokhar Tsarnaev was apprehended on a boat parked on a residential property in Watertown, Massachusetts. The bombing, on April 15 at the finish line of the marathon, killed three people and wounded at least 170. (Photo by Jared Wickerham/Getty Images) ( 167263249 )

A woman wearing a Boston Marathon runner's jacket watches as police officers file into St. Patrick's Church for the funeral of Massachusetts Institute of Technology (MIT) police officer Sean Collier in Stoneham, Massachusetts April 23, 2013. Officer Collier was allegedly killed by the brothers accused of the Boston Marathon bombings, Dzhokhar and Tamerlan Tsarnaev. Two explosions hit the Boston Marathon April 15, killing at least three people and injuring over 100 others. REUTERS/Brian Snyder ( USA-EXPLOSIONS/BOSTON )

Mourners hug as they depart St. Patrick's Church in Stoneham, Mass., following a funeral Mass for Massachusetts Institute of Technology police officer Sean Collier, Tuesday, April 23, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Steven Senne) ( Boston Marathon Officer Funeral )

Mourners depart St. Patrick's Church in Stoneham, Mass., following a funeral Mass for Massachusetts Institute of Technology police officer Sean Collier, Tuesday, April 23, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Steven Senne) ( Boston Marathon Officer Funeral )

A mourner walks out of St Patricks Church following the funeral for the slain Massachusetts Institute of Technology (MIT) police officer Sean Collier at St Patrick's Church on April 23, 2013 in Stoneham, Massachusetts. Collier was allegedly shot by the suspects in the Boston Marathon bombing, Dzhokhar A. Tsarnaev, 19, and his brother Tamerlan Tsarnaev, 26, after their identities were determined and a manhunt was launched. Tamerlan Tsarnaev was subsequently shot and killed after a car chase and shootout with police and Dzhokhar Tsarnaev was apprehended on a boat parked on a residential property in Watertown, Massachusetts. The bombing, on April 15 at the finish line of the marathon, killed three people and wounded at least 170. (Photo by Jared Wickerham/Getty Images) ( 167266791 )

A mourner reacts as she departs St. Patrick's Church in Stoneham, Mass., with others following a funeral Mass for Massachusetts Institute of Technology police officer Sean Collier, Tuesday, April 23, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Steven Senne) ( Boston Marathon Officer Funeral )

Mourners walk past police motorcycles as they depart St. Patrick's Church in Stoneham, Mass., following a funeral Mass for Massachusetts Institute of Technology police officer Sean Collier, Tuesday, April 23, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. A truck that belonged to Collier sits behind. (AP Photo/Steven Senne) ( Boston Marathon Officer Funeral )

Richard Corapi holds up an American flag as the limousine carrying family and friends drives through town following the funeral for the slain Massachusetts Institute of Technology (MIT) police officer Sean Collier near St Patrick's Church on April 23, 2013 in Stoneham, Massachusetts. Collier was allegedly shot by the suspects in the Boston Marathon bombing, Dzhokhar A. Tsarnaev, 19, and his brother Tamerlan Tsarnaev, 26, after their identities were determined and a manhunt was launched. Tamerlan Tsarnaev was subsequently shot and killed after a car chase and shootout with police and Dzhokhar Tsarnaev was apprehended on a boat parked on a residential property in Watertown, Massachusetts. The bombing, on April 15 at the finish line of the marathon, killed three people and wounded at least 170. (Photo by Jared Wickerham/Getty Images) ( 167266788 )

A tribute honoring fallen Massachusetts Institute of Technology police officer Sean Collier is seen on a window of a truck that belonged to Collier as it departs St. Patrick's Church in Stoneham, Mass., following a funeral Mass, Tuesday, April 23, 2013. Collier was fatally shot on the MIT campus Thursday, April 18, 2013. Authorities allege that the Boston Marathon bombing suspects were responsible. (AP Photo/Steven Senne) ( Boston Marathon Officer Funeral )