FBI publishes photos of Boston Marathon bomb suspects

Investigators searching for the Boston Marathon bombers released dramatic photographs and surveillance video footage last night of two men carrying backpacks they suspect of planting the two bombs on Monday.

The suspects were shown on the footage walking through spectators on the street of the near-simultaneous blasts which killed three and injured more than 170 people in the worst attack on US soil since September 2001.

The Federal Bureau of Investigation pleaded the public for help in trying to identify the suspects at a press conference in Boston late yesterday evening but warned that they should not approach the pair or try to apprehend them.

The two suspects, described as “people of interest” to investigators, are considered “armed and extremely dangerous”, said Rick DesLauriers, the FBI chief in charge of the bombing investigation.

The two men were shown in footage walking at the corner of Gloucester Street and Boylston Street, just over a block away from the second blast near the finish line of one of the world’s most famous marathons.

The second suspect, who wore a white cap, dark jacket, hoodie and dark trousers, is suspected of leaving the bomb at the scene of the second blast at the Forum restaurant on Boylston Street, about 100 metres from the first nearer the finish line.

Investigators had previously said that the bombs may have been planted in backpacks and at least one may have been packed with metal shrapnel in a crudely-made pressure cooker-type kitchen pot.

The first significant breakthrough in the three-day investigation came just hours after US President Barack Obama promised a shattered Boston that those responsible for Monday's marathon bombings would be hunted down.

Drawing on his strength as an orator, the president - on his first visit to the city since the attacks - gave an emotional speech at a church service, saying that the people of Boston and the US won’t “cower in fear” over the attack and that their resilience was “the greatest rebuke to whoever committed this heinous act”.