Midwest City, Oklahoma (CNN) It was such a quiet Monday afternoon on the streets of this Oklahoma City suburb that David Huff was thinking of sneaking in some sunset fishing before heading home.

But just after 3 p.m. on June 17, 2013, the call shot across Midwest City Police Department radios. Across the street from the police station, in a Walmart grocery store, a man had snatched a young girl and was holding her at knifepoint in front of her mother and stunned shoppers.

"All the people that are first responders, we all know that in our mind. We know that the next call could be the one that changes your life. It could be the next thing that ends your life," police Maj. David Huff said.

Huff and Detective Darrell Miller, trained hostage negotiators, raced out of the station and within minutes came face to face with Sammie Wallace, who was holding a knife to 2-year-old Zoey Keating's stomach.

Huff, 50, has rarely spoken publicly about the dramatic events that unfolded during a volatile 30-minute hostage standoff. During an emotional interview with CNN, his eyes often welled with tears.

He talked about dealing with the "tough stuff," admitting that he still beats himself up, struggling with the horror of what he was forced to do.

"There is a lot of tough stuff to deal with. Taking someone else's life. Cop for 26 years, and I never fired a shot at another human being. Ever," said Huff.

Huff hopes his story and his journey over the past three years will help other police officers learn to cope with the stress of what they confront on any day in the line of duty.

Huff was one of 13 police officers awarded the Medal of Valor by President Obama on May 16 for his "uncommon poise to end the result of a dangerous hostage situation."

June 17, 2013

After shoppers' 911 calls poured in, Huff and Miller calmly walked into a terrifying scene.

"We wanted to keep him calm," Huff said. "He was in this manic state where he was basically blurting out statements."

Huff and Miller suspect that Wallace was mentally ill, which made persuading him to let Zoey go difficult. They say Wallace was highly agitated. Surveillance video captured Wallace wandering the store for several minutes, pushing an empty grocery cart, before grabbing Zoey from her mother's cart.

Congrats, MWC Major Huff! Medal of Valor recipient @ White House. See video of award here: https://t.co/FoouDakxhI pic.twitter.com/JS2ejRXILK — Midwest City PD (@MidwestCityPD) May 16, 2016

Alicia Keating, Zoey's mother, said it was "unreal." She couldn't believe her eyes when she saw Wallace holding a knife.

"I just stared at him like he was crazy," Keating said. "When he started to walk away with Zoey, that's when I started screaming, 'Somebody help! This man has my baby!' "

Just two minutes after the initial 911 calls, the first police officer arrived on the scene, walking back to the frozen meat section to assess the situation and help keep Wallace cornered.

Shortly after, Huff and Miller arrived and immediately went to work.

The store was cleared of shoppers, and Zoey's mother was moved away. It was now Wallace holding Zoey, talking to Huff and Miller alone.

They tried various techniques to get Wallace to calm down and release the girl. They brought him a chair and tried to reason with him, but he was frequently incoherent. Tension was escalating. Miller described him as getting more and more "amped up" as the conversation deteriorated.

"When he started to speak, and he started making statements about the cameras are all watching us, it's the Illuminati. And George Bush is watching us. And he made reference to the CIA and that type of thing," Huff said.

"We got the feeling right off the bat it was probably somebody who was mentally ill. And to go even further, those types of statements we knew usually come from someone who has paranoid schizophrenia."

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About 34 minutes after Huff and Miller arrived, the situation unraveled. Wallace threatened to end Zoey's life, demanding that the officers bring in a Dallas police officer with whom he went to school. As his demands weren't met, he started a 60-second countdown, moving Zoey closer to his chest and the knife closer to her throat.

"Looking at the way he was acting and the things he was doing, I absolutely took him at his word that when he got down to zero, he would hurt Zoey," Huff said.

"I had it in my head that when he gets close to five, I'm going to take action. I said it in my head, the only way Zoey doesn't get hurt is if the barrel of my gun is touching his head."

When Wallace counted down to five, Huff said, he wouldn't wait any longer. "I had to take one large step and had to do it," Huff recalled.

On the surveillance video, just as he had planned, Huff swiftly moves the gun right behind Zoey's body and fires one shot directly into Wallace's head. Wallace collapses in the chair. In an instant, a team of officers converges and scoops Zoey from Wallace's arms. He was bleeding from the head and was still holding the knife in his left hand, according to the police report.

The video then shows Huff spinning away in a rush of emotion. He leans over the shelves of the meat aisle, punching the wall. The reality that his horrific moment is finally over sinks in.

"I didn't know if he had already stabbed her," an emotional Huff said of Zoey. "When I picked her up, my intention was that I was going to run her back to the ambulances. Darrell grabbed her from me without missing a beat and ran her back."

'He's my superhero'

For the first year after the incident, Huff said, he did the "cop thing" and pretended that "nothing bothered him."

"The things you see and are exposed to, you kind of put that in a box in the back of your mind and you say, 'OK, that's the bad stuff. We're not going to think about that.' "

Huff said that being honest and going through the emotions was key to him moving past his frequent feelings of darkness.

He said other officers told him he handled the situation perfectly, but in Huff's mind, a perfect situation would have been everyone walking out of the store alive, even Wallace.

"There's a feeling of failure. I know in my logical mind that taking his life most likely saved Zoey's life. And in my logical mind, I know that is the choice that had to be made. But you still have this feeling that you failed, because if you had succeeded, you would have thought of some way to do this without taking a life."

It took him about a year to want to watch the surveillance video. He credits openly discussing the day with Miller and others with helping end his internal struggle.

"I was angry. I wasn't angry at Sammie Wallace. I was angry that he made me do it. And at that point, I thought that Zoey had already been cut or stabbed and that I waited too long," Huff said, wiping away tears. "They finally came around and said, 'She doesn't have a scratch on her.' That was the best part."

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Alicia Keating, Zoey's mother, heard the gunshot inside the store and feared the worst. She then saw Miller racing toward her with Zoey covered in blood. Zoey was not injured, and Keating felt an overwhelming sense of relief.

Three years later, she's grateful for officers like Huff.

"I saw for myself, firsthand, how scary their job must be," Keating said. "And how brave they are."

Zoey, now 5, calls Huff her superhero who "saved the day."

"Don't tell Zoey this, but I'm not a superhero. But hearing it from that beautiful child, I'm telling you, that right there will make your chest stick out for sure, " Huff said with a huge smile.

"It wasn't really any type of hero deal. It was a chain of a bunch of people being involved in a divine intervention. Because there were a bunch of heroes in the room that day."