(CNN) -- President Obama brought a mixture of reassurance and defiance to Boston on Thursday to help heal a city hit hard by terrorist bombs.
"Every one of us stands with you," the president said at an interfaith service inside the Cathedral of the Holy Cross. "Boston may be your hometown -- but we claim it, too. ... For millions of us what happened on Monday is personal."
Then Obama's tone took a more defiant turn toward those who planted the two bombs that exploded near the Boston Marathon's finish line Monday. "Yes, we will find you. And yes, you will face justice," Obama said. "We will hold you accountable."
Calling the event a chance to "mourn and measure our loss," the president also reaffirmed that Boston's spirit remains "undaunted and the spirit of this country shall remain undimmed." He looked ahead to next year's race, predicting that "the world will return to this great American city to run even harder and to cheer even louder for the 118th Boston Marathon. Bet on it."
The audience responded with excited applause.
"Your resolve is the greatest rebuke to whoever committed this heinous act," the president told those at the service. "If they sought to intimidate us, to terrorize us, to shake us from those values that (Massachusetts Gov. Deval Patrick) described, the values that make us who we are as Americans, well, it should be pretty clear by now that they picked the wrong city to do it."
Other dignitaries attending the service included the first lady Michelle Obama; the president's former election rival, Mitt Romney; Gov. Patrick; and Boston Mayor Thomas Menino. During an interlude, attendees were soothed by a performance by famed cellist Yo-Yo Ma. About 2,000 people filled the ornate cathedral, The Boston Globe reported, with about half the seats reserved for the public.
Under heavy security, the audience also included scores of police officers and other first responders. After the service as people exited the church, crowds erupted in cheers, while others sang the national anthem. Later, the president stopped at a high school to thank a group of first responders and volunteers, and the first lady met with patients, families and hospital staff at Boston Children's Hospital and Brigham and Women's Hospital, the White House said.
Meanwhile, behind the scenes, investigators were combing through surveillance video dating to at least a week before the bombings to try to identify anyone who walked the finish-line route before the race, said a source who receives regular updates on the investigation. Authorities were working around-the-clock work to identify two men that a law enforcement source told CNN are pictured in images captured before the blast -- not far from the race's finish line, one of them lugging a black backpack.
It was in such a backpack that investigators believe the bomber or bombers placed explosive devices that killed three and wounded more than 180 Monday toward the end of the Boston Marathon.
"Every hour we're closer," Patrick told CNN's "Situation Room" on Wednesday. "And I say that because we've got the very best professionals at every level working this. And working it hard."
At Thursday's service, Boston's mayor praised each of the three bystanders who were killed in the blasts -- Martin Richard, described as a "young boy with a big heart"; Krystle Campbell, whose spirit "brought her to the marathon year after year"; and Lingzu Lu, who "came to the city in search of an education."
On Wednesday, a law enforcement official who is being regularly briefed on the investigation told CNN's Susan Candiotti that images showing two men near the marathon finish line were being circulated to state and federal law enforcement agencies. The source described the men as "possible suspects."
But a source told CNN's Deb Feyerick on Thursday that those individuals are no longer of high interest to investigators.
The men faced scrutiny and were considered potentially important to the case because one of them had been carrying a black backpack close to one of the bombing sites.
Asked about the images at a congressional hearing Thursday, Homeland Security Secretary Janet Napolitano urged caution.
"There is some video that has raised the question of those that the FBI would like to speak with," she said. "I wouldn't characterize them as suspects under the technical term. But we need the public's help in locating these individuals."
She said the investigation is "proceeding apace."
But she said, "This is not an 'NCIS' episode. Sometimes you have to take time to properly put the chain together to identify the perpetrators, but everyone is committed to seeing that that gets done in the right way."
Earlier Wednesday, two sources with knowledge of the investigation identified a man as a possible suspect in the attack but did not name him.
Seen on a video, the man wore a white baseball cap. One of the sources added that the cap was on backward and the man was also wearing a light-colored hooded sweatshirt and a black jacket.
It was not immediately clear if he is one of those alluded to in the photographs distributed to law enforcement officials.
The reports came after a chaotic day in which some law enforcement sources initially told media outlets that a suspect had been arrested, only to have the FBI and Boston police issue formal denials that any suspect was in custody.
Patrick urged patience to allow investigators space to do their job.
"I wish they had nailed the perpetrator within minutes of this catastrophe, but I understand from experience it's going to take some time," he said.
Details of bombs
Investigators say the bombs, which exploded 12 seconds apart, were designed to deliver the most vicious suffering.
One was housed in a pressure cooker hidden inside a backpack, the FBI said. The device also had fragments that may have included nails, BBs and ball bearings, the agency said.
The second bomb was in a metal container, but it was unclear whether it was in a pressure cooker as well, the FBI said.
Photos obtained by CNN show the remains of a pressure cooker found at the scene, along with a shredded black backpack and what appear to be metal pellets or ball bearings.
They were sent to the FBI's national laboratory in Virginia, where technicians will try to reconstruct the devices.
In the past, the U.S. government has warned federal agencies that terrorists could turn pressure cookers into bombs by packing them with explosives and shrapnel, and detonating them with blasting caps.
While the clues moved the investigation forward, it is still unclear whether the attack was an act of domestic or foreign terrorism.
Authorities sifted through thousands of pieces of evidence and a mass of digital photos and video clips. They have pleaded for the public's help in providing additional leads and images.
More than 60 people remained hospitalized Wednesday night in Boston-area medical centers, 13 of them in critical condition, according to hospital officials.
The three dead include:
• Richard, the 8-year-old boy with a gap-tooth grin and bright eyes. He loved to run and play in his yard.
• Campbell, a 29-year-old freckle-faced woman described by her mother as having "a heart of gold."
• Lu, the Chinese graduate student at Boston University who had moved to the city last fall, making friends and soaking up new experiences.
The U.S. State Department has been in contact with her family and the Chinese Foreign Ministry, Secretary of State John Kerry said in a statement released Thursday.
"We stand ready to provide whatever appropriate assistance we can to the family members of foreign nationals in the aftermath of this despicable act of terror," Kerry said.