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She had just put her 9-month-old down for a nap, turned on cartoons for the older kids and was headed for the dishwasher when she heard a strange "pop" come from the bedroom of the Missouri home.

Alexis Wiederholt, 26, said that as she rushed to investigate the noise, her 5-year-old son appeared and said something that didn't make any sense to her in the moment.

"I'm sorry, Mom. I shot Corbin."

Weiderholt ran past to the pack-and-play where Corbin, her always-smiling youngest, should have been resting peacefully.

"I walked in and there was my baby, lying there, bleeding," the young mother told NBC News, her voice cracking as she described the Monday morning scene.

"I had just hugged him in my arms five minutes before that."

In what police have said was a tragic accident, the 5-year-old had gotten hold of his grandfather's .22 caliber Magnum revolver and fired a shot that struck Corbin in the head, mortally wounding him.

As she grieves for her baby and worries about the future of her eldest son, whose name is being withheld by NBC News, Wiederholt said her loss should be a warning to others to protect children from firearms.

She said she had no idea that her father, William Porter, kept a loaded pistol in the Elmo home she and the children were visiting.

"I didn't know it was there until I turned around and saw it laying on the bed," she said. "I don't know why someone would have a loaded gun in the house while kids were around."

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"I'm sorry, Mom. I shot Corbin."

Porter told NBC News that he had the pistol for security and target practice and kept it in a locked case in his bedroom with other guns he uses for sport.

"I set it up behind the long arms. It's always been there. I've never moved it," he said.

It's unclear how the child got into the case, but Porter said it could be opened with a screwdriver or even a random key.

"I told the boys they weren’t supposed to be in my bedroom where I keep the gun cabinet and they knew it — but like I said, boys will be boys," Porter said in an emotional phone interview.

Asked whether he regrets keeping the weapons in the home, Porter said, "I do now."

The Nodaway County Sheriff is continuing to investigate but said "foul play is not suspected."

Sheriff Darren White said he's a supporter of gun ownership — but that too many people are complacent about weapons.

"There are a lot of people who have handguns for home protection who should maybe rethink that," he said. "And when you have kids running around, you don't leave loaded handguns anywhere."

Investigators have not yet interviewed the 5-year-old, and his mother said he hasn't processed his little brother's death.

"His dad tried talking to him about it, but he doesn't understand," Wiederholt said, adding that she still doesn't know why he fired the gun.

"Maybe he thought it was something off a TV show," she said.

"I don't know why someone would have a loaded gun in the house while kids were around."

Wiederholt, who lives in North Dakota with her husband, said she brought the kids to her father's house near the Iowa border so she could report for a National Guard stint.

"When I was growing up we always had guns in the house," she said. "We grew up on a farm and we hunted all the time.

"That's why I can't believe that the man who taught me gun safety — you keep guns unloaded and keep the safety on and keep them locked up — had a gun in the house with my children there."

Porter broke down in tears as he talked about the damage his gun had caused.

"Right now, I'm going through hell," he said. "We are all taking it hard."

"He was a doll," he said of Corbin. "He was the sweetest thing you ever put your eyes on."