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PHOTO GALLERY: Boston on alert as US police hunt for marathon bombing suspect



Police officers aim their weapons Friday in Watertown (Photo: AP)
Police officers walk near a crime scene Friday in Watertown (Photo: AP)
Police work a crime scene Friday (Photo: AP)

Police in tactical gear conduct a search for a suspect in the Boston Marathon bombings AP
Police in tactical gear conduct search for marathon bombing suspect (Photo: AP)
Residents view police in tactical gear conduct search for marathon bombing suspect (Photo: AP)
SWAT teams enter neighbourhood to search an apartment for remaining suspect (Photo: Reuters)
SWAT teams search for remaining marathon bombing suspect (Photo: Reuters)
Police in tactical gear surround an apartment building while looking for suspect (Photo: AP)
Police officers take position during search for suspect (Photo: Reuters)
Federal Bureau of Investigation shows a suspect that officials identified as Dzhokhar Tsarnaev AP