Obama addresses memorial service in Boston
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Boston Marathon: Obama vows bombers will face justice

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

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

He spoke as it emerged that investigators had 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.

Mark Mardell reports.

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