Investigators have released pictures of two suspects in the Boston bombing, seeking the public's help in finding two men photographed on the crowded pavement before Monday's bombs exploded near the finish line.
Three people were killed and 176 were injured in the attacks.
Both men carried backpacks that are believed to contain the bombs.
The man identified as suspect number one wore a dark baseball cap.
Suspect number two wore a white cap backwards and was seen setting down his backpack on the ground,
FBI Special Agent in Charge of the investigation Richard DesLauriers said: "Somebody out there knows these individuals as friends, neighbors, co-workers or family members of the suspects.
Though it may be difficult, the nation is counting on those with information to come forward and provide it to us," he said.
The bombings began a week of security scares that rattled the United States and evoked memories of the 11 September 2001 attacks.
Obama attends memorial service
President Barack Obama sought to bring solace to Boston and the nation in an interfaith service at a cathedral about 1.6km from the bomb site, declaring "you will run again" and vowing to catch whoever was responsible.
He promised resilience in a message directed toward Boston but also to a country that was on edge.
A massive explosion at a fertiliser factory devastated a small Texas community, sending shockwaves at least 80km (50 miles) away.
Some of the victims of the Boston attack suffered gruesome injuries, and at least ten lost limbs as a result of the blasts.
Investigators believe the bombs were made of pressure cookers packed with shrapnel.
"As you begin this long journey of recovery, your city is with you, your commonwealth is with you, your country is with you," Mr Obama said.
"We will all be with you as you learn to stand and walk and, yes, run again. Of that, I have no doubt. You will run again."
After his speech, Mr Obama met with volunteers and Boston Marathon organisers, many of whom cared for the injured, and with victims at Massachusetts General Hospital.