A doctor was crossing the finish line of the Boston Marathon when she heard the blasts which killed three people.
With no thought for her own safety, Dr Natalie Stavas instinctively ran towards the victims and began to treat the wounded.
In the minutes after the explosions, she carried out CPR and used bits of clothing to help stop people's wounds bleeding.
Now, a week on, the pediatrician is coming to terms with what happened and the impact it will have on the rest of her life.
'As a physician, I take an oath to do the best I can,' she told the LA Times.
'I've carried with me this intense sorrow and intense grief since that day.'
The doctor attended a memorial service this week and met some of the other first responders.
She said: 'I was able to sit with the Boston police and be in the presence of true heroes surrounded by truly heroic people who were, as a collect as a whole, were able to save so many lives.'
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Dr Stavas was running the race with her father, who is also a doctor, when she reached the end of the 26-mile route.
Her father Joe, 58, a radiologist, said: 'She was like an Olympic runner - I couldn't keep up with her.'
Medics: Dr Natalie Stavas and her father had reached the end of the marathon when they heard the explosions
The daughter of a doctor and nurse, Dr Stavas grew up in Nebraska and was always destined to go into medicine.
She is now a pediatric resident treating children at the Boston Medical Centre and Boston Children's Hospital.
But the medic was nearly forced to pull out of the marathon because she broke her left foot a couple of weeks before the race.
Determined: Dr Natalie Stavas says she now hopes she can move on from the bombings after treating victims
Her determination however, spurred her on to take part because she was doing it for charity.
Had she not been there, victims of the blasts would not have been given treatment so quickly.
She said: 'Finally I'm starting to have a sense of peace and a sense of closure and an urgent need to be able to move beyond my grief and move towards the future.
'I hope I can help us move forward and be a stronger person and overcome what we have been through...and go back out there next year and cross the finish line of the Boston Marathon.'
Dr Natalie Stavas was crossing the finish line of the Boston Marathon when the first bomb went off