Only two explosive devices were involved in an “act of cowardice” that left at least 176 people injured and three dead near the finish line of the 117th Boston Marathon yesterday, city officials said today. “Make no mistake, an act of cowardice and of this severity can not be justified or explained; it can only be answered,” Suffolk County District Attorney Daniel Conley said during a press conference at the Westin Copley Plaza Hotel today. “The loss we’ve suffered is enormous.” Authorities said the explosions were caused by two devices. No unexploded devices were found, Gov. Deval Patrick said. Other...