President Barack Obama told mourners in Boston today that everyone had been touched by the attack on their beloved city.
Speaking at an interfaith memorial service, Mr Obama declared "there is a piece of Boston in me" and said a day of beauty was shattered when a celebration became a tragedy.
The President said Boston gathered "to reaffirm that the spirit of this city is undaunted" and promised: "You will run again!"
His remarks at the service came amid a manhunt for at least two men seen on video taken before two blasts struck near the finish line on Monday, killing three and wounding 176 in a crowd of tens of thousands.
Homeland Security Secretary Janet Napolitano confirmed today that the FBI was searching for people seen on a video taken near the finish line.
Of the perpatrator, Mr Obama said: "We will find you."
Dozens of uniformed police officers in bright yellow vests formed a security line around the church. City officials used buses to block access to nearby cross streets.
"There is some video that has raised the question of those that the FBI would like to speak with," Napolitano said in Congressional testimony today. "I wouldn't characterize them as suspects under the technical term. But we do need the public's help in locating these individuals."
Boston Mayor Tom Menino, Massachusetts Governor Deval Patrick and Cardinal Sean O'Malley were also scheduled to speak at the service.
Menino was one of the first to speak.
"This is Boston, a city with courage, compassion and strength that knows no bounds," said Menino, who was rolled to the podium in a wheelchair but stood for his remarks despite breaking a leg over the weekend. "We love the brave ones who felt the blast and still raced through the smoke with ringing in his ears ... to answer cries of those in need."
Obama was also due to meet families of victims of the bombing and first responders while in Boston, a White House spokesman told reporters aboard Air Force One.
The service comes the day after the FBI arrested a Mississippi man in connection with letters believed to have contained the deadly poison ricin and sent to federal officials, including Obama. In a separate incident yesterday, an explosion at a Texas fertilizer plant killed as many as 15 people.
The FBI said there was no indication of a connection between the ricin letters and the Boston bomb attacks, but they reminded Americans of anthrax mail attacks in the wake of the September 11 suicide hijackings 12 years ago.
Earlier, hundreds of people crowded outside the Cathedral of the Holy Cross in Boston's South End, about a mile from the bombing site.
"President Obama knows how important the city of Boston is to the nation and the world," said 55-year-old John Snyder, who had joined the line before sunrise. "He is bringing his light to us for much-needed healing."
Investigators believe the Boston bombs were fashioned out of pressure cookers and packed with shrapnel. Ten victims lost limbs, and emergency room doctors reported plucking nails and ball bearing from the wounded.
Police had considered making an appeal to the public for more information at a news conference yesterday, a US government source said, but the FBI cancelled it after a number of delays.
Boston Police and FBI officials said today that they had not determined whether they would publicly release more details of the investigation.
The bombs in Boston killed an 8-year-old boy, Martin Richard; a 29-year-old woman, Krystle Campbell; and a Boston University graduate student and Chinese citizen, Lu Lingzi.
Ahead of his visit, Obama declared a state of emergency in Massachusetts, a move that makes federal funding available to the state as it copes with the aftermath of the bombing.
The crowded scene along the race course in central Boston on Monday was recorded by surveillance cameras and media outlets, providing investigators with significant video footage of the area before and after the two blasts.
Based on the shards of metal, fabric, wires and a battery recovered at the scene, the focus turned to whoever may have placed homemade bombs in pressure cooker pots and taken them in heavy black nylon bags to the finish line of the world-famous race.
Tens of thousands of people turn out to watch and run in the marathon, which comes on a state holiday and is one of New England's best-attended sporting events.