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An eight-year-old boy waiting at the finish line to give his father a hug was among three people killed and more than 144 injured when two explosions rocked the Boston Marathon in the worst terrorist atrocity since 9/11.

Martin Richard died after bombs hidden in rubbish bins were detonated within seconds of each other during the Patriots’ Day bank holiday yesterday, sending terrified runners and spectators fleeing for their lives and leaving a scene of ‘unspeakable horror’.

Boston police closed off the city as a ‘danger zone’ as witnesses described seeing body parts flying through the air and shoes that ‘still had flesh in them’ in an attack that has sent shockwaves across the world.

At least 17 of the injured are in a critical condition and it is thought as many as 30 victims may have needed amputations or major corrective surgery on maimed limbs.

At least eight of the wounded are thought to be children, including a two-year-old boy who suffered a head injury.

Another was 11-year-old Aaron Hern, 11, of Martinez, California, who was hit by flying shrapnel in his thigh as he waited for his mother to cross the finish line. He is being treated at Boston Children’s Hospital and is expected to undergo further surgeries.

Today, as the United States remained on full alert, President Barack Obama vowed to hunt down the culprits and ‘hold them accountable’.

Startling video footage showed an explosion going off in the heart of the crowd that had lined the streets of the Massachusetts city yesterday to watch the famous sporting event.

After the twin detonations ripped through the cheering crowds, one witness told CNN that it ‘felt like a huge cannon’ and other described horrifying scenes of screaming spectators, missing limbs and unresponsive bodies.

‘In 28 years, this is definitely the worst I’ve seen,’ said District Fire Chief Ron Harrington of the Boston Fire Department’s District 3 to NBC News.

‘Bodies and body parts. Blood all over. A little boy lying in the street. A young woman in her twenties. Both dead. It was mayhem. I saw two people with arms hanging loose, and one without a leg. A shoe with flesh still in it.’

After the blasts, two more suspicious devices were found in the city’s Mandarin Oriental and Lenox hotels. Both were evacuated and the suspected bombs dismantled.

Read more: Dailymail