US President Barack Obama has told a memorial service for victims of the Boston marathon bombing the attack's perpetrators will be held accountable.

He told an interfaith service that everyone had been touched by the attack on their beloved city.

His visit comes as investigators found clear video images of two potential suspects carrying black bags.

Two bombs were detonated near the finish line on Monday, killing three people and injuring more than 170.

Roads were closed around the Cathedral of the Holy Cross, the main Roman Catholic cathedral in Boston, ahead of the president's arrival on Thursday.

FBI caution

A heavy police presence was seen in the area as residents waited - some arriving before dawn - for one of about 2,000 seats inside.

Before his visit, the president declared a state of emergency in Massachusetts, which would allow the state to access federal funding to deal with the aftermath of the attacks.

President Obama appeared sombre as he joined city leaders, residents and victims at the prayer service.

He told the congregation: "The spirit of this city is undaunted; the spirit of this country shall remain undimmed."

He was applauded as he warned the perpetrators of the attacks they would be brought to justice.

"We will find you," he said. "We will hold you accountable."

Afterwards, he was expected to meet emergency workers and the families of victims, a White House spokesman said.

Former Republican presidential challenger Mitt Romney also attended the prayer ceremony.

On Thursday Homeland Security Secretary Janet Napolitano said the FBI wanted to speak to two men, but did not say they were suspects.

"There is some video that has raised the question of those that the FBI would like to speak with," Ms Napolitano said.

"I wouldn't characterise them as suspects under the technical term, but we do need the public's help in locating these individuals."

She did not give details of what was seen on the tape.

Ms Napolitano would not say whether the attacks were suspected to be the work of a domestic or foreign group, but said the investigation was continuing "apace".

She added that security had been ramped up at airports and transport hubs.

The FBI has denied a flurry of reports on Wednesday that a suspect had been arrested.

Investigators are going through thousands of images taken around the time of the attacks from security cameras, the media and people at the scene.

Officials have not said if they would publicly release more details of the case.

Correspondents say authorities are cautious about publicising the relevant images, after the release of pictures relating to a 1996 bombing at the Atlanta Olympics led to the arrest of an innocent security guard, Richard Jewell.

Officials said a circuit board and battery pack - parts of a triggering mechanism - had been recovered and the lid of a pressure cooker, apparently blown off during the explosion, was found on the roof of a nearby building.

The bombs are believed to have consisted of explosives placed in pressure cookers, one with shards of metal and ball bearings, the other with nails.

Nobody has claimed responsibility for the bombs, which a source said had been placed in black bags and left on the ground.

The twin blasts killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, a postgraduate student from China.

Shards and shrapnel

Dozens remain in hospital following the bombs, many of them seriously injured.

It is reported that 10 victims have had limbs amputated. Nails and ball bearings were said to be embedded in their flesh.

At least 14 bombing victims, including three children, remain in hospital in a critical condition.

The first explosion went off close to the finish line at about 14:50 local time (18:50 GMT) on Monday.

Seconds later, as rescuers were rushing to help the injured, another explosion went off nearby.

The London marathon - the next major international marathon - is to go ahead on Sunday.

Organisers have said they will hold a 30-second silence at the start as a mark of respect.