. A Boston police officer stands guard at a memorial site at Boylston and Arlington streets along the course of the Boston Marathon on April 16, 2013, a few blocks from where two explosions struck near the finish line of the Boston Marathon on April 15. The explosives used in the Boston Marathon bombings were likely homemade devices full of nails and metal fragments designed to cause widespread injury, according to initial reports. A day after an attack that left three dead and more than 170 wounded, the FBI and Boston police declined to reveal details of their probe, or whether they suspected the assault was linked to foreign or domestic extremists.   DON EMMERT/AFP/Getty Images
{"Image ImageDescription": "TOPSHOTS\nA Boston police officer stands guard at a memorial site at Boylston and Arlington streets along the course of the Boston Marathon on April 16, 2013, a few blocks from where two explosions struck near the finish line of the Boston Marathon on April 15. The explosives used in the Boston Marathon bombings were likely homemade devices full of nails and metal fragments designed to cause widespread injury, according to initial reports. A day after an attack that left three dead and more than 170 wounded, the FBI and Boston police declined to reveal details of their probe, or whether they suspected the assault was linked to foreign or domestic extremists.   AFP PHOTO/Don EmmertDON EMMERT/AFP/Getty Images", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image YCbCrPositioning": "Centered", "Image ExifOffset": "818", "EXIF ExifVersion": "0230", "EXIF ComponentsConfiguration": "YCbCr", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB"}