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Darko Vojinovic, Associated Press
A runner shows a banner reading: "Boston we are with you - Belgrade runners" in an organized memorial run to show solidarity with victims of the Boston Marathon bombing, Tuesday, April 16, 2013, in Belgrade, Serbia. The explosions Monday afternoon killed at least three people and injured more than 140.

Heartbreak Hill is only supposed to be marathon mile 21. But on April 15, 2013, heartbreak was the culminating experience for Boston's most famous race.

It was Patriot's Day, an especially meaningful holiday in Boston, when a picture-perfect Marathon Monday ended with three confirmed dead, including an 8-year-old boy from Dorchester. More than 140 were reported injured, many critically. More details will come, but we do know this: All of us have witnessed a tragedy.

At 2:50 p.m., just after the four-hour mark for the third wave of runners, two explosions went off near the finish line on Boylston Street by the viewers stands. The blasts blew out the windows of the Marathon Sports Store.

Instead of euphoria and a finishing time, there were stretchers, blood, horrific wounds, tears and chaos. Police officers ran into the race to stop runners from coming any closer to possible danger.

Reporting live, CBS Boston reporter Jonathan Elias could not believe what he was seeing. "This just doesn't happen here," he said, fighting back tears.

Boylston is the final stretch of the Boston Marathon, where runners are exhausted but exultant; the finish line in sight. And since Boston is a qualifying race, it is that much more legendary. For many, it's a once-in-a-lifetime-dream. Runners from around the world come for their Boston moment. Some have spent years planning; all have spent months training.

Cyndi Lubrano of New Hampshire finished the race eight minutes before the first explosion. She was still in the finishing chute picking up her medal, blanket and bag when she heard what "sounded like a canon." Turning, she saw smoke. "It was reminiscent of 9/11," she said. With cell coverage turned off, no one knew what was going on, but this also helped quell panic. When emergency vehicles began rushing past, Lubrano immediately thought, "I have to get to my family!" With transportation shut down, Lubrano began walking.

Boylston is highly secure before every Boston marathon, with bomb-sniffing dogs walking the streets before the race. The problem is, like at any race, after the start, people can come and go as they please. Fans line the streets the entire 26 miles, making the Boston crowd deservedly well-loved. But close and cheering crowds also make it impossible to know what is carried in from the outside.

Boston marathoner Maryn Barrett, of New Hampshire, ran a great race and was on her way home when she heard of the explosions. She immediately checked in with her running friends, relieved to hear they were okay. Barrett said, "It is a terrible, tragic thing — the day went from one of celebration to one of complete sadness." Her niece hopes Barrett will never run the race again. "I am not sure how I feel about that," Barrett said. "But I know that it won't ever be the same."

Runners and married couple Sarah Lester and Brian Reynolds of New Hampshire were also in Boston on race day. Lester was a spectator this year, while Reynolds ran. Neither of them wanted to talk much about it. Lester said, "I will say we will definitely be back next year where I have zero doubt the Boston Marathon will be better than ever." She said she saw acts of kindness they would not have usually seen. "My thoughts go out to the families affected."

In the 8:45 p.m.evening press conference, Dan Conley, Suffolk district attorney, spoke of the hundreds of volunteers that immediately reached out and helped others. "That's what Americans do in times of crisis. We come together and help each other." And that's what they did. Eric Adelson of Yahoo! Sports wrote of the tremendous outpouring of thousands of city residents who indeed became patriots, offering what they had to any displaced runners needing help.

Kathleen Nimmo works in the marketing department at Mass General and had just left work when the bombs went off. Only 24 years old, Nimmo is also president of her church's young adult women's group. Upon hearing the news, Nimmo immediately thought of her girls working in Boston. But "the phones were shut down because they thought that's how the bombs were being detonated so I couldn't call anyone," Nimmo said. She began to text, sending multiple messages over and over again until she received a personal response. "Every single person texted me right away; they knew it was important. They knew they would be missed if they didn't respond."

Lester echoed, "On the bright side, I got to see what good people are capable of." She also added, "The Boston Athletic Association has done a fabulous job."

Runners are resilient folks. They know the highs and lows of running, the wins and the losses, the great finishes and painful injuries. They are prone to draw parallels to life. From what we know of the runners, today they are broken but unbowed. Today there is shock and sadness. Tomorrow they will lace up their running shoes.

Sore and stiff from having to walk so far after the race, Lubrano found her family and headed home. She said the tragedy wouldn't change her mind about Boston again. Lubrano is a runner. "I'm sure, next year, at the finish line it will feel like déjà vu." But, she added, "Runners are tough people, aren't they?"

Amy Makechnie is a runner and writer from New Hampshire. She is the author of www.maisymak.com