The first Boston Marathon bomb exploded about two hours and forty minutes after the winner of the men’s race had crossed the finish line on Boylston Street. I was standing on the finish line photographing runners; some dressed in costumes, others were holding the hands of children and family members as they shared the joy of finishing together.

The first bomb went off on the Boylston Street sidewalk less than 45 feet from me. The percussion from the blast jolted me. I saw runner Bill Iffrig from Lake Stevens, Wash., fall to the pavement. I ran forward to photograph him. Three Boston Police officers bolted towards him at the same time, one with her gun drawn, as the second bomb exploded three blocks away. I had not seen the officer's gun until I edited the photos. I didn’t understand at first what had happened, thinking maybe it was a cannon salute or a manhole explosion. But when I ran to the sidewalk area several feet away, I saw the horror of what the bomb did. I then began to comprehend what I was photographing. I tried not to take my eye off the camera. Smoke was thick. A police officer looked me in the eyes and said, “You shouldn’t be here. Another bomb could go off.” Bodies were still smoldering, legs were blown off, and massive amounts of blood covered the sidewalk.

I photographed Kevin Corcoran from Lowell using his belt to stop the flow of blood from his wife Celeste’s mangled leg. I photographed Celeste’s daughter, Sydney, lying on the ground as two men wrapped t-shirts on her legs. Nicole Gross from Charlotte, North Carolina, struggled to get up, her shirt in tatters, her leg severely injured. A pool of blood surrounded her. A police officer reached over and placed two fingers on Krystal Campbell's neck, looking for a pulse. Krystal had passed away. Celeste lost both legs. Sydney lost nearly all her blood from a cellphone-size piece of shrapnel that lodged in her thigh and severed her femoral artery. I continued taking photos for about 12 minutes. Then I got my laptop, which was still plugged in near the finish line, and walked to my car and drove to the Globe office. I took off my shoes, which were covered with blood, and began to edit my photos. On bad nights, the images still play over and over in my mind.