At a press conference late Thursday afternoon local time, FBI agent Richard Deslauriers said that the Bureau was seeking the public's help to locate the two suspects. He said that the FBI only believed that footage showed one of the men planting one of the bags that investigators believe eventually exploded. 

"We consider them to be armed and extremely dangerous," Deslauriers said. "It continues to be an ongoing, active investigation," he added.

The crudely fashioned bombs were said to have been made with ordinary kitchen pressure cookers, explosives, nails and ball bearings. The blasts killed 8-year-old Martin Richard, from Boston, 29-year-old Krystle Campbell, from nearby Medford, and Lu Lingzi, a graduate student from China who was attending Boston University.

'The greatest rebuke'

In a commemoration service on Thursday, US President Barack Obama warned that the perpetrators they would be held to account.

The president visited Boston's Roman Catholic Cathedral of the Holy Cross to take part in an inter-faith service to commemorate victims of the bombing. Aside from the three people killed, some 170 other people at the marathon were left injured, 10 of them losing limbs.

Having paid tribute to those who had died, Obama told the congregation that Boston's spirit as a city would help it to recover.

"You will run again because that's what the people of Boston are made of," Obama said.

"Your resolve is the greatest rebuke to whoever committed this heinous act," he added, warning that justice would be meted out to those involved in the attack.

"We will find you and we will hold you accountable," he said. Obama added that America would not bow to those who threatened a "free and open society."

"We may be momentarily knocked off our feet, but we'll pick ourselves up and we'll keep going. We'll finish the race," Obama said.

mkg/ipj (AFP, Reuters, dpa, AP)