Barack Obama pays tribute to city

President Barack Obama speaks at a memorial service for the victims of the Boston Marathon bombings.

BOSTON -- 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.

"Every one of us stands with you," Obama said Thursday, speaking at an interfaith service in the aftermath of Monday's twin blasts that killed three and injured more than 170 people at the finish line of the Boston Marathon.

Obama said a day of beauty was shattered when a celebration became a tragedy.

"You will run again!" he declared.

Of the perpetrator, he said: "We will find you."

Investigators for a second day are on the hunt for a man seen in a department-store surveillance video dropping off a bag at the site of the bombings, a Boston politician said.

Separately, a law enforcement official confirmed that authorities have found an image of a potential suspect but don't know his name.

Wednesday's development marked a possible turning point in a case that has investigators analyzing photos and videos frame by frame for clues to who carried out the twin bombings and why.

City Council president Stephen Murphy, who said he was briefed by Boston police, said Wednesday that investigators saw the image on surveillance footage they got from a department store near the finish line, and matched the findings with witness descriptions of someone leaving the scene.

"I know it's very active and very fluid right now -- that they are on the chase," Murphy told The Associated Press. He added: "They may be on the verge of arresting someone, and that's good."

The bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings, investigators and others close to the case said. Investigators suspect the devices were hidden in black duffel bags and left on the ground.

As a result, they were looking for images of someone lugging a dark, heavy bag.

One department store video "has confirmed that a suspect is seen dropping a bag near the point of the second explosion and heading off," Murphy said.

The FBI and other law enforcement officials are planning a news briefing Thursday afternoon on the investigation.

Information from The Associated Press was used in this report.

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