President Barack Obama told a heartbroken Boston "you will run again," three days after bombs ripped through the finish line crowd at the city's famous marathon, killing three people and injuring 170.
The president spoke Thursday at an interfaith service in Boston.
"We will all be with you as you learn to stand and walk and, yes, run again," Obama said. "Of that I have no doubt, you will run again."
Boston Mayor Thomas Menino said at the service that "nothing will take us down, because we take care of one another."
The blasts killed 8-year-old Martin Richard, of Boston, 29-year-old Krystle Campbell, of Medford, and Lu Lingzi, a Boston University graduate student from China. At least 14 bombing victims, including three children, remain in critical condition.
The service came as the painstaking work to identify a bombing suspect from reams of Boston Marathon footage yielded a possible breakthrough.
NBC News reported that authorities discovered an image showing a man dropping off a bag then walking away from the site of the second of two deadly explosions.
The footage hasn't been made public, but the FBI distributed a photo to other law enforcement agencies showing the man, who was wearing a white or off-white baseball cap and was talking on a cell phone, a senior federal official who had seen the photo told NBC News. Investigators have combed cellphone records in an effort to identify him.
The federal official described the man as about 6 feet tall and said a second person seen with him was also being sought.
The FBI might release the footage later Thursday, NBC News said.
As of Thursday morning, there were no claims of responsibility for the attacks or conclusion among authorities over whether they were acts of domestic or international terrorism, according to NBC News.