Kelvin Ma is a photographer for the Bloomberg Photo Service. He was stationed about 50 feet from the finish line at the Boston Marathon as the explosions occurred. Ma took the photo of wheelchair-bound Jeff Bauman moments after he looked into the eyes of the man who tried to kill him.  Bauman’s written words, “bag, saw the guy, looked right at me,” helped identify who perpetrated one of the highest-profile acts of terror in the U.S. since September 11, 2001.

Kelvin shares his experience below in an edited blog post:

What happened on Monday is not something you mentally prepare for.

The first blast occurred less than a minute after I had photographed a Tufts runner crossing the finish. There was some confusion as to what had happened. I thought it was a cannon, much like we hear every Fourth of July at the Boston Pops concert on the Esplanade. No one seemed quite sure what had happened.

Then the second blast went off, farther down Boylston Street. Pandemonium erupted as spectators knocked over the crowd-control barriers and scattered.

I made a few pictures of the injured emerging from the smoke on wheelchairs. I couldn’t see much through the haze, so I ran up the photo bridge, suspended above the finish line, to get a better view.

At heart, I am a photojournalist, having worked for newspapers and wire services for the better part of the last 10 years. We often ask ourselves how we would react in a tough situation, because invariably, we end up covering fires, shootings and other tragedies. This was different, though, and I had no idea what I would see from the top of the bridge.

Right away I knew it was bad. I saw people with horrendous injuries, one in particular that was difficult for me to look at, or photograph. The man, later identified as Jeff Bauman, had lost both legs below the knee and was being carted away on a wheelchair as the now well-known peace activist Carlos Arredondo sprinted beside him, applying tourniquets to both limbs.

The response from emergency personnel and bystanders was impressive, a description that even now seems meager, given what they did. Those who ran toward the blast scene, without giving thought to their own safety, easily outnumbered the victims, three to one.

Every single one of those people is my hero.

We were evacuated off the bridge shortly thereafter, at which point I texted my mom that I was OK and sent out a two-word Tweet—“I’m OK.”—for everyone else. I then contacted Bloomberg News, and the photo editor, Graham Morrison, encouraged me to find a place to sit down, regain my composure and transmit the images I had captured. I was running on pure adrenaline and had no idea what to do, but Morrison’s reassurance and counseling through all of the trauma and chaos helped me stay grounded in the situation.

After securing an internet connection in the Prudential Center, I pushed my photos to the wire. I sent more emails and made more social media posts to let people know I was OK, and then headed back toward the scene, making some more photos along the way.

The outpouring of support from the photojournalism community has been incredible, whether it be from people I’ve known for years to someone I just met on Monday. I can’t express enough how grateful I am to have such supportive brothers and sisters in my corner every step of the way.

I tracked down the last runner I photographed before the blast and gave her a hug. Down to a matter of seconds, it really was a miracle she was standing there unscathed. I have never been so happy to see someone I hadn’t met yet.

Around 9 p.m. Tuesday, I saw a familiar face on the New York Times website. Jeff Bauman, whose horrific injuries had kept me awake the night before, had survived and was stable in a local hospital. Learning that he had made it lifted the heavy cloud that had engulfed me. His recovery will no doubt be long and difficult, but he is still with us.

What struck me most about the horror of the situation was not the devastation and the chaos, but the fact that so many people ran toward the danger to help those in need without giving a second thought about themselves. That seems to be a recurring thread in everyone’s recollection of what happened, and it can’t be repeated enough.

As a member of the media, I will inevitably receive negative comments and criticism about the photos that I took, because some people disagree about what the public should and should not be able to see. I understand that. But as a professional witness, I don’t know how else to show not only the evil of the world, but also the compassion and humanity that ultimately overcomes it, like the actions of the first responders and volunteers who dove headfirst into the smoke to save so many lives on Monday.

Kelvin Ma, Photographer for the Bloomberg Photo Service