No one knows that better than people who were on the scene, such as Franciscan Father Brian Jordan, a chaplain at St. Francis College in Brooklyn Heights, and Bridget Cunningham, the niece of Father Jim Cunningham, who is pastor of Holy Name Parish in Windsor Terrace.
Father Jordan, who has run the Boston race 21 times, was sidelined with a knee injury this year. He made the trip to support the many friends he has made there in previous years.
He celebrated an outdoor Mass for about 100 Boston firefighters and state police at the start of the race in Hopkinton, Mass. He then took the train to the finish line, and it was while he was arriving there that the explosions went off.
"I heard the explosions. I didn't see them," the priest told The Tablet, newspaper of the Brooklyn Diocese. "I thought it had something to do with the Patriot's Day celebration.
"But then I saw the horrible sight of people suffering, running from the scene. It was a terrible scene of blood and confusion, and I knew it was an attack on the people of Boston."
Cunningham, who attends every year as a guest of the marathon organization, said she heard nothing. She was in the Copley Plaza Hotel, the race headquarters, on the third floor. She found out about the disaster from Facebook. The hotel was immediately put into lockdown, forcing her to spend an unplanned evening in Boston.
"I didn't want to go outside," she said. "The whole evening it was very quiet, very sad, just heartbreaking. Usually the elite runners are talking about their times and wondering if they could have done better. This time, it was as if the race never happened."
Father Jordan had the presence of mind to run back to nearby St. Anthony's Shrine to retrieve his Franciscan habit in order to identify himself.
"People identify with the Franciscan habit. It's an international sign of peace and consolation," he said. He tried to calm the people who were confused. They were asking what had happened. Had there been other explosions?
"There was a grave sense of uncertainty," he said, as people were searching for their husbands, wives and children.
The Franciscan, who also was a first responder during the Sept. 11 terrorist attacks a dozen years ago, praised the immediate response of the local medical teams. He explained that because the incident occurred near the finish, many medical personnel were on hand to assist runners with bruises and other discomforts.
"The medical people, they were the first responders," said Father Jordan. "Then there were the police who also did a great job.
"I'm not a medical person or a police officer; I'm just a Franciscan priest. I just went up to people and talked to them. I tried to give a message of serenity and peace, a message of hope."
Later that evening, Father Jordan visited local hospitals.
When he returned to New York the following morning, there was a sense of heightened security at the airport. Law enforcement officers were asking people at the airport if they had any photos or film of the Marathon that might help in the investigation. It was all done with great courtesy and professionalism, he pointed out.
By April 18, the FBI had identified two brothers from the Russian region of Chechnya as suspects in the bombings, which killed three people and left between 160 and 170 injured. The two men terrorized the Boston area overnight April 18. One was shot dead by police, but by midday April 19 the other remained on the loose, police said, and was considered armed and dangerous.
Father Jordan, a man of hope, predicts the Boston Marathon will return bigger and better than ever.
Cunningham will be there. "It's tradition," she said. "I can't imagine not being there."