US President Barack Obama landed in Boston on Thursday, on his way to a memorial service for victims of the Boston Marathon bombing amid a manhunt for two suspects seen on video taken before two blasts struck near the finish line on Monday.

The president and First Lady Michelle Obama attended a special inter-faith service at Boston's Cathedral of the Holy Cross with victims of Monday's double bomb attack, which killed three people and wounded 176. No arrests have yet been made in the investigation.

US President Barack Obama and First Lady Michelle Obama board Air Force One at Andrews Air Force Base in Maryland on Thursday. Obama was in Boston to mourn victims of the deadly marathon attacks, as investigators study images of a suspect who may have planted the bombs. Jewel Samad / Agence France-Presse

"For millions of us, what happened on Monday was personal," the president said at the service.

Obama spoke of each of those killed in the bombings at the marathon on Monday.

He said the nation's prayers are with the family in China who sent their daughter to Boston University "so she could experience all this city has to offer".

"She was a 23-year-old student, far from home. ... With the tears of her friends and family we are reminded of this tragedy."

A line of people several city blocks long formed outside the cathedral in Boston's South End, about 1.6 km from the bombing site.

"President Obama knows how important the city of Boston is to the nation and the world," said 55-year-old John Snyder, who had joined the line before sunrise. "He is bringing his light to us for much-needed healing."

Security was heavy around the church, with dozens of city police officers and bomb-sniffing dogs. Police were also out in force around the bombing site, which remains closed off for several blocks as they search for evidence.

Video shows suspects

Authorities have clear video images of two separate suspects carrying black bags at each explosion site and are planning to release the images in an appeal for the public's help in identifying the men, according to an official briefed on the case.

The official said that the two suspects were seen separately on videotape - one at each of the two bombing sites, which are located about a block apart.

Wednesday's developments marked a possible turning point in the case that has investigators analyzing photos and videos frame by frame for clues to who carried out the twin bombings and why. The footage hasn't been made public.

City Council President Stephen Murphy said investigators saw the image on surveillance footage they got from a department store near the finish line, and matched the findings with witness descriptions of someone leaving the scene.

"I know it's very active and very fluid right now - they are on the chase," Murphy said. He added: "They may be on the verge of arresting someone, and that's good."

The bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings. Investigators suspect the devices were then hidden in black duffel bags and left on the ground.

As a result, they were looking for images of someone lugging a dark, heavy bag.

One department store video "has confirmed that a suspect is seen dropping a bag near the point of the second explosion and heading off", Murphy said.

The turn of events came with Boston in a state of high excitement over conflicting reports of a breakthrough.

A law enforcement official briefed on the investigation said around midday that a suspect was in custody. The official, who was not authorized to divulge details of the investigation and spoke on condition of anonymity, said the suspect was expected in federal court. But the FBI and the US attorney's office in Boston said no arrests had been made.

AP-Reuters -AFP