BOSTON – John Simonson, of Fargo, had just finished the Boston Marathon with a time of 3 hours, 56 minutes Monday afternoon. He collected his blanket and a finisher’s medal, then headed to a bus about 300 yards from the finish line to put on long pants. He waved to his wife, Kelley, and sister-in-law, Lynee Arnold, who were nearby. Then he heard the blast. It was so strong it seemed to reverberate off the skyscrapers of Boston’s core and back down to the asphalt street, then through Simonson’s body. He could see white smoke lifting from the ground.