John Byrnes was swimming in a pool beneath Hunter College in Manhattan on the morning of Sept. 11, 2001, when he stepped outside for coffee and heard sirens blaring down Lexington Avenue. He learned that a plane had hit the World Trade Center. Minutes later, he saw the North Tower collapse on a campus television and immediately headed toward his National Guard armory. Byrnes spent the next two weeks helping secure Ground Zero. And spent the foll…