The president also held up Bill Iffrig, the Lake Stevens marathoner whose image was captured as he lay crumpled on the ground after the first blast, as an example of the American spirit.
"Like Bill Iffrig, 78 years old, the runner in the orange tank-top who we all saw get knocked down by the blast, we may be momentarily knocked off our feet, but we'll pick ourselves up," Obama said.
"We'll keep going. We will finish the race."
Obama addressed an interfaith service in the aftermath of Monday's twin blasts that killed three and injured more 170 people at the finish line of the Boston Marathon.
Obama said a day of beauty was shattered when a celebration became a tragedy.
He said Boston gathered Tuesday, quote, "to reaffirm that the spirit of this city is undaunted."
He declared: "You will run again!"
Of the perpetrator, he said: "We will find you."
Obama spoke as his second-term as president is increasingly burdened by terror, politics, and disaster. In the aftermath of Boston's deadly blasts Monday, Obama lost a fight for gun control measures in the Senate, was the target, along with a U.S. senator, of letters that showed traces of poisonous ricin, and awoke Thursday to news of a powerful fertilizer plant explosion that devastated a small Texas town.
The letters alone -- one addressed to Obama and another to Sen. Roger Wicker, R-Miss. -- evoked eerie parallels to the anthrax attacks that followed the terrorist attacks of Sept. 11, 2001. Authorities arrested a Mississippi man Wednesday in connection with the letters.
It was against that backdrop that Obama and his wife, Michelle, came to Boston Thursday morning, joining a crowd at the Cathedral of the Holy Cross for a "Healing Our City" service. The Obamas sat at the front of the church next to Massachusetts Gov. Deval Patrick as the service began.
Obama listened from his pew as Boston Mayor Thomas Menino praised the response of his city.
"Nothing will take us down because we take care of one another," Menino said. "Even with the smell of smoke in the air and blood in the streets and tears in our eyes, we triumphed over that hateful act."
Moments later, Patrick said: "We will grieve our losses and heal. We will rise, and we will endure. We will have accountability without vengeance, vigilance without fear."
Sustaining that uplifting theme, Obama recalled his days as a law student at Harvard and declared, "There is a piece of Boston in me."
"Every one of us has been touched by this attack on your beloved city, every one of us stands with you," he said.
Obama also planned to meet with some of those injured, as well as the first responders who rushed toward the blast to help the scores of runners and spectators.
White House spokesman Josh Earnest said Obama received a briefing from national security adviser Lisa Monaco on the status of the investigation into the Boston blast before departing the White House. Accompanying Obama aboard Air Force One were members of the Massachusetts congressional delegation, including Sens. Elizabeth Warren and Mo Cowan.
"We send our support and encouragement to people who never expected that they'd need it -- the wounded civilians who are just beginning what will be, I'm sure for some of them, a long road to recovery," Obama said Wednesday in a likely preview of his remarks at the service.
The president has stepped into this role as the nation's consoler in chief many times before in his presidency, most recently in December after the massacre of 20 first-graders and six educators at an elementary school in Newtown, Conn. Before that, there were the deadly shootings in Aurora, Colo., Tucson, Ariz., and Fort Hood, Texas, as well as the natural disasters that tore apart towns and neighborhoods in Missouri and the New York-New Jersey area.
This time, Obama must confront the unique challenges of a terror attack that inevitably revived memories of 9/11. As he did in a statement from the White House on Tuesday, the president was expected to urge the public to remain vigilant, while declaring that "the American people refuse to be terrorized."
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