Detectives investigating the deadly Boston marathon bombings released the first CCTV footage of two suspects.
The images showed two men, one in a black baseball cape a second in a white baseball cap, and both wearing backpacks as they walked along a pavement next to the race.
FBI special agent Richard DesLauriers appealed for the public’s help in identifying the suspects but warned: “We consider them to be armed and extremely dangerous.”
He told a press conference in the city: “At this time these are the two people of interest to the FBI.
“The only photos that should be officially relied upon in this investigation are those you see before you today. There’s no additional imminent danger that we’re aware of right now.”
Nobody has claimed responsibility for the twin explosions which injured more than 180 people and killed eight-year-old Martin Richard, Krystle Campbell, 29, and a Chinese graduate student, named in reports as Lu Lingzi.
The images were released hours after President Barack Obama attended an interfaith service in Boston to remember the victims.
There was confusion on Wednesday after several media outlets reported that a suspect was in custody and was due to appear in court over the atrocity, but investigators later urged caution over the reports and insisted no one had been arrested.
As well as claiming Martin’s life, the explosion reportedly tore off one of his younger sister Jane’s legs and delivered a blow to the head that gave his mother Denise, 43, serious brain injuries.
In a statement, his father Bill said: “My dear son Martin has died from injuries sustained in the attack on Boston. My wife and daughter are both recovering from serious injuries.
“We thank our family and friends, those we know and those we have never met, for their thoughts and prayers. I ask that you continue to pray for my family as we remember Martin.”
Miss Campbell, 29, from Arlington, Massachusetts, who was watching her boyfriend run the marathon, was named as another spectator killed in the attacks.
Miss Campbell’s father William described his daughter, who worked for a restaurant consultancy firm, as a “wonderful, wonderful girl”.
He told reporters: “She helped everybody, and I’m just so shocked right now. We’re just devastated.
“She was a wonderful, wonderful girl. Always willing to lend a hand.”
The Chinese consulate in New York confirmed that the third victim was one of its citizens, a Boston University graduate student, named by Chinese media as Lu Lingzi.
There have been no claims of responsibility for the attack so far, the most serious in the US since the 9/11 atrocity. The man in the white cap was seen setting down a backpack at one site near the finish line, Mr DesLauriers said.
The bombs were made from ordinary kitchen pressure cookers packed with metal and ball bearings. Investigators suspect the devices were then hidden in duffel bags and left on the ground.
They exploded within 15 seconds of each other near the finishing line as thousands of runners were pouring in.
At the service, Mr Obama called the perpetrators of the attack “these small, stunted individuals who would destroy instead of build”.
He has called the marathon bombings an act of terrorism. His visit to Boston came after it emerged he was sent a letter apparently containing poisonous ricin.
There was no indication the letter, which was intercepted by the White House, was connected to the marathon bombings, the FBI said.
The Boston Marathon is one of the world’s oldest and most prestigious with about 23,000 runners taking part, including hundreds from the UK and Ireland.