President Barack Obama to Speak at Boston Service

(CNN) -- Boston will pause to mourn, and heal, Thursday after the fatal terror attack on the city's beloved marathon.

President Barack Obama, who on Wednesday signed an emergency declaration allowing for expanded federal support for Boston, will be among the speakers at a public interfaith prayer service in the city's Cathedral of the Holy Cross.

Meanwhile, behind the scenes, hundreds of investigators will continue their 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," Massachusetts Gov. Deval 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."

Working leads

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 photographs are not being released to the public for fear of impeding the investigation, the source said.

The source described the men as "possible suspects."

In the images, the source said, one of the men is seen carrying a black backpack.

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."

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.

Casualty update

More than 60 people remained hospitalized Wednesday night in Boston-area medical centers, 13 of them in critical condition, according to hospital officials.

Three people died:

• Martin Richard, an 8-year-old boy with a gap-tooth grin and bright eyes. He loved to run and play in his yard.

• Krystle Campbell, a 29-year-old freckle-faced woman described by her mother as having "a heart of gold."

• The third victim was Lingzu Lu, a 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.

'Make room for love'

The emotions are still raw: anger, confusion, terror.

But those who lived through the twin blasts at the marathon -- and those touched by the tragedy -- are slowly picking up the pieces and trying to move on.

The city will make a formal effort of that Thursday with the planned 11 a.m. interfaith prayer service featuring Obama, Patrick and Cardinal Sean O'Malley, among others.

It's time to "make room for love, said Lisa Conti, a graduate student in Boston.

"Imagine how much deeper the wound would be if we filled the empty spaces with hate instead of love."

But not everyone is ready, or able, to move on. For Candace Rispoli, who witnessed the bombing, the pain was still too raw.

"My hands have still not stopped shaking," Rispoli said.

CNN's Tom Watkins, Susan Candiotti, Henry Hanks, Fran Townsend, Matt Smith, Dave Alsup, Henry Hanks, Rande Iaboni, Gloria Borger and John King contributed to this report.