A look at the deadly Boston Marathon bombing - Article Photos 2/6

An investigator in a protective suit examines debris on Boylston Street in Boston Thursday, April 18, 2013 as investigation of the Boston Marathon bombings continues. Investigators pressed the search Thursday for one or more potential suspects spotted on video, while President Barack Obama paid a visit under heavy security to offer reassurance to the city and a warning to those responsible for the attack: "We will find you." (AP Photo/Elise Amendola)