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BOSTON (AP) -- President Barack Obama declared there is a piece of Boston in me as he paid tribute to a city shaken by what he has called an act of terror. He said: Every one of us stands with you.

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.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

President Barack Obama sought to soothe a nervous nation Thursday and draw attention to those killed and gravely wounded in what he called an act of terror at the Boston Marathon.

Americans also looked to the president to offer reassurances about the nation's safety as investigators scrambled to answer key questions about an attack with origins that are yet unknown.

The president was to speak at an interfaith service in Boston honoring the three people killed and 170 injured when a pair of bombs ripped through the crowd gathered Monday afternoon near the finish line of the famous race. Investigators had an image of a potential suspect, though much about what happened remained a mystery, keeping tensions high in Boston and elsewhere around the country.

Compounding the nation's jitters were letters sent to Washington officials that contained suspicious substances, including ones addressed to Obama and Sen. Roger Wicker, R-Miss., that showed traces of poisonous ricin in initial tests. The letters evoked eerie parallels to the anthrax attacks that followed the terrorist attacks of Sept. 11, 2001.

It was against that backdrop that Obama and his wife, Michelle, arrived in 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.

White House spokesman Josh Earnest told reporters traveling with the president on Air Force One that Obama also planned to meet with some of those injured, as well as with the first responders who rushed toward the blast to help the scores of runners and spectators.

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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