April 15, 2017 marked four years since the Boston Marathon bombing. Three people died and more than 260 wereinjured when two homemade bombs packed with nails and screws were detonated in the crowds of spectators, a short distance from the finish line.

Among those seriously injured in the bombing was 26-year-old Rebekah Gregory, who was standing near the finishing line with her five-year-old son, Noah. She has told the story of her life and faith in Taking My Life Back, published today in the UK. This is an extract used with permission.

The blast wave hit like a freight train and I felt as if the street itself had exploded. All of a sudden I found myself thrown back, sprawled on the ground, like something out of a movie. What? What just happened? Somehow I wasn't knocked out, but the power of the blast flattened me so hard and fast that it had me in a world of its own. A deafening clang reverberated in my head. Smoke filled the air. I managed to slightly lift my head, enough to see my body covered in blood. Bits of flesh, including pieces of my own leg bones, were plastered all around me.

I raised my gaze a little higher and saw other victims. There was no telling how many. Pieces of metal were scattered everywhere. Random body parts were strewn among the bodies themselves. The entire finish line was no longer recognizable. We were in a war zone. But as horrific as that scene was, my true nightmare came a few moments later when my mind cleared enough to remember that Noah had been sitting on my feet with his back against my legs. Now my legs were shredded and I couldn't see my son anywhere.

I tried to scream for someone to find him, but I could barely hear myself or anything else over the gong reverberating in my head. Then a few moments after the first explosion (I didn't know until later that the interval was 12seconds), the second bomb went off. Now the pandemonium was thick. Panicked bystanders and baffled officials struggled to respond. From the corners of my limited field of vision, I saw the yellow jackets of race officials swarming the scene, joined by a few of the spectators. Some of those who had escaped injury were attempting to provide first aid or perhaps comfort the dying. I mustered any physical strength that was left in me and tried to scream again for my son, for Noah, but everyone within earshot was either injured, stunned, or preoccupied with first aid and essential medical help.

At that point my initial shock subsided enough for the pain to come roaring in. It was paralyzing, unlike anything I've known. The pain also made it clear that this thing might have already killed me. My instincts seemed convinced that I would be pulled down the same road as the nearby silent victims unless I fought back.

An instinctive force compelled me to find Noah and start taking care of things, but at the same time I realized this was a futile impulse. My legs were destroyed and my little boy had vanished from my sight. Instinct kept telling me I was in danger of bleeding out, but I didn't want to know anything else except what had happened to him. The prospect of leaving this world before I could do anything to help him was unbearable. I'm certain every parent feels something similar for their child in dire circumstances. I had to know what had happened to him. No matter what the truth was, I had to know.

My eardrums felt as if they had been blown out by the blast (and it turned out they had). The gong still reverberated deep in my skull and threw up a din that drowned out nearly everything else. The throbbing pain was matched by the burning stabs from the shrapnel punctures all over my body. Shrapnel also covered everything else around me, as if a cloud of metallic insects had died in flight and fallen straight to the ground.

By the time I noticed my clothing was also smouldering, one of the first responders had run over and began pulling the burning clothes away from me. He put his forehead next to mine and shouted loudly enough for me to hear him, assuring me I would be taken care of. I couldn't make coherent responses, but I understood he was there to give me a chance at life. He was doing this even though the second explosion had made it plain nobody was safe. For all he knew, a third bomb and a fourth bomb were about to go off, taking him out along with me.

While I lay there, barely able to see, barely able to hear, I got the distinct impression of Noah's voice hollering, 'Mommy! Mommy!' from somewhere close behind me. I was able to twist my head to see him on the ground a few yards behind me. My first reaction was to stretch out my arms to pull him toward me, even though he was too far away for that to be possible. My friend's aunt was nearby, unhurt. She and some first responders picked Noah up and began to tend to him. Amazingly, he hadn't been badly injured. His leg was bleeding, but it appeared he wasn't damaged anywhere near the extent that I was. It made no sense to me that he was so free from injury, but gratitude filled my heart anyway. Out there on that street, I felt my priorities shift the instant Noah's survival was confirmed; I could let my own condition become important. Unless I could somehow rally, my son would have been spared from the explosion only to watch me die on that pavement.

An emergency medical technician leaped from an ambulance that was suddenly close by. I could faintly hear him shouting right next to me, 'We need to get her off the street or she'll die here!' At the rate I was bleeding, I had only a few minutes left. I glanced up at the guy who was loading me into the ambulance just in time to see him yell to the driver, 'We have an amputee...' His voice barely penetrated the ringing in my ears, but I read his lips and was alert enough to understand him. Even if I survived, life as I knew it was over.

'Taking My Life Back: My Story of Faith, Determination, and Surviving the Boston Marathon Bombing' by Rebekah Gregory with Anthony Flacco is published today in the UK, price £14.99 (hardback). It's also available as an e-book.