Hours earlier, he was a happy 4-year-old who loved Ironman and the Hulk and all the Avengers. Now, as Bryson Mees-Hernandez approached death in a Houston hospital room, his brain swelling through the bullet hole in his face, his mother assured the boy it was OK to die.

“When you are on the other side,” his mother, Crystal Mees, recalls telling him, “you are going to see Mommy cry a lot. It’s not because she’s mad. It’s because she misses you.”

And this: “It’s not your fault.”

But whose fault was it?

Bryson shot himself last January with a .22-caliber Derringer his grandmother kept under the bed. It was an accident, but one that could be blamed on many factors, from his grandmother’s negligence to the failure of government and industry to find ways to prevent his death and so many others.

The Associated Press and the USA TODAY Network set out to determine just how many others there have been.

The findings: During the first six months of this year, minors died from accidental shootings — at their own hands, or at the hands of other children or adults — at a pace of one every other day, far more than limited federal statistics indicate.

Tragedies like the death of Bryson Mees-Hernandez play out repeatedly across the country. Curious toddlers find unsecured, loaded handguns in their homes and vehicles, and fatally shoot themselves and others. Teenagers, often showing off guns to their friends and siblings, end up shooting them instead.

Using information collected by the Gun Violence Archive, a nonpartisan research group, news reports and public sources, the media outlets spent six months analyzing the circumstances of every death and injury from accidental shootings involving children ages 17 and younger from Jan. 1, 2014, to June 30 of this year — more than 1,000 incidents in all.

Among the findings:

—Deaths and injuries spike for children under 5, with 3-year-olds the most common shooters and victims among young children. Nearly 90 3-year-olds were killed or injured in the shootings, the vast majority of which were self-inflicted.

—Accidental shootings spike again for ages 15-17, when victims are most often fatally shot by other children but typically survive self-inflicted gunshots.

—They most often happen at the children’s homes, with handguns legally owned by adults for self-protection. They are more likely to occur on weekends or around holidays such as Christmas.

—States in the South, including Alabama, Louisiana, Mississippi, Tennessee and Georgia, are among those with the highest per capita rates of accidental shootings involving minors.

In all, more than 320 minors age 17 and under and more than 30 adults were killed in accidental shootings involving minors. Nearly 700 other children and 78 adults were injured.

The U.S. Centers for Disease Control and Prevention reported that 74 minors died from accidental discharges of firearms in 2014, the latest year for which comparable data are available. The AP and USA TODAY analysis counted 113 for that year, suggesting the federal government missed a third of the cases.

While accidental shootings account for only a fraction of firearm deaths in the U.S., gun safety advocates have long argued that they are largely preventable and thus prefer to call them unintentional shootings, rather than accidental.

“The extent of the problem is a little bit shocking. The extent of the undercount is a little bit shocking,” said Lindsay Nichols, an attorney at the Law Center to Prevent Gun Violence in San Francisco. “A lot of it provides further evidence that this is such a horrible pattern that continues and that more action is needed.”

Gun control advocates demand stricter laws requiring guns to be kept locked up and unloaded. But gun rights supporters argue those measures make guns less useful in emergencies; citing CDC statistics, the National Rifle Association argues in public statements that such deaths have declined significantly in recent decades and that the chance of a child dying in a firearms accident is “one in one million.”

Bob Anderson, chief of the mortality statistics branch of the CDC’s National Center for Health Statistics, suggested the NRA was citing statistics that underestimate the risk guns represent to American children.

He would not, he said, “put money on that interpretation.”

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Bryson and his 2-year-old sister were staying with their grandparents in January to give their mother a break. She had given birth months earlier to a baby girl and needed sleep.

It was a typical night. After their baths, Bryson asked if he could hop in bed with his grandfather, who was already asleep. His grandmother, Anna Sperber, said yes, before she fell asleep on the living room couch with the younger girl.

When Sperber got up to get a blanket hours later, she saw Bryson face down on the bedroom floor. She thought he had fallen asleep.

Then she noticed the lump on his head and panicked when she saw the pistol she kept under her bed lying next to him. He had shot himself above the right eye.

Bryson’s grandfather, who used a noisy breathing machine, had slept through the gunshot. After a frantic 911 call, emergency responders would take Bryson to a hospital by helicopter as his sister watched. Soon, a neurosurgeon would tell his mother it was only a matter of time before the boy would be dead. The bullet had gone straight to the back of his brain and shattered.

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The undercount documented by the AP and USA TODAY Network is “significant and important,” but not surprising, said the CDC’s Anderson.

He said the agency has long suspected that its statistics on accidental firearms deaths are too low; the agency tracks deaths from accidental discharges of firearms but tallies only those that are officially classified that way by coroners on death certificates. Coroners categorize many such deaths as homicides because they can fit the definition of being killed by another. They also can classify them as undetermined, if the cause is unclear.

The AP and USA TODAY Network did not rely on coroners’ findings, but rather counted those shootings that were declared accidental or unintentional by investigating agencies. They were tallied primarily from the Gun Violence Archive, where researchers track media, government and commercial sources to compile a comprehensive database of gun incidents. The review also uncovered a handful of shootings that had not previously been made public.

The findings were in line with prior research done by Everytown for Gun Safety, the advocacy group founded by former New York City Mayor Michael Bloomberg, which keeps a running database of such shootings. The group says that up to 70 percent of accidental shootings could be prevented if parents kept their guns locked and separated from ammunition, as recommended by the American Academy of Pediatrics.

Since the start of 2014, more than 80 children age 4 and under have fatally shot themselves.

“That 3- to 5-year-old age group, they are going to be looking at the gun when they shoot it. They point the gun barrel at themselves and put their thumbs on the trigger,” said Sheriff David McKnight of Marion County, Texas, who is investigating the death of a 3-year-old who killed himself with his father’s gun in July.

But children of every age, and especially boys, seem drawn to guns.

Last year, 4-year-old Cameron Price and his 6-year-old brother, Ka’Darius, were riding their bikes outside the Levingston Motel in Shreveport, Louisiana, where their family had taken a $30-a-night room — all they could afford, their mother would later say.

They decided to go inside, into a room where several adult acquaintances of their parents had been smoking marijuana.

A gun was sitting out, and Ka’Darius thought the chrome and black .40-caliber pistol was a toy.

A single shot rang out. Robert Price found his younger son slumped over on the arm of a couch when he entered after hearing a loud “pop” while in the bathroom of an adjacent room. He cradled the toddler as he took his final breaths before being whisked to a hospital, where he died.

Ka’Darius later told police he “pushed the bad button” and he understood his brother “had a hole in his head,” was going to the hospital and not coming home.

The Caddo Parish district attorney’s office charged two people in the room with weapons and drug charges. Both pleaded down to attempted possession of a firearm by a convicted felon. In August, a judge sentenced them to four years with credit for time served.

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For children ages 10 and up, the cases usually involve a group playing together when one manages to obtain a gun.

The AP-USA TODAY Network research showed the most vulnerable years are the middle teens, specifically for those just reaching the age when they can obtain a driver’s license.

A shooting in Iowa earlier this year illustrates one of the more common types of accidental shootings.

It was the Friday of spring break, and the University of Iowa Hawkeyes had just won an NCAA tournament game on a buzzer-beater. It was a thrill for Senquez Jackson, a basketball-loving 15-year-old from Cedar Rapids.

He relayed the news of the big win to his mother, who was resting in her bedroom before her night shift at an educational testing company.

The high school freshman returned to the living room to hang out with friends, who had spent the week with him and his older brother, Malik.

One, a 17-year-old, had brought over a handgun that had been legally owned by a friend’s mother and recently disappeared from her home. A 13-year-old wanted to see the weapon and removed the clip. Thinking it was unloaded, he started playing with it.

“Bro, I’m hit,” Senquez said.

The friend thought he was kidding, but the gun had fired. The bullet ripped through the right side of Senquez’s chest, piercing his lung.

His mother, Tamara Bloemendaal, was jolted awake with Malik screaming: “Mom, get up. Senquez has been shot.”

Bloemendaal recalls helping Senquez out of the recliner and watching him collapse on the floor in a pool of blood. She rode in the ambulance with the tall, handsome boy she called “Chunks” as a baby. Within hours, he was dead.

The 13-year-old pleaded guilty to involuntary manslaughter in juvenile court, expressing remorse.

“It doesn’t seem like any of them had access to a gun before. They were fascinated by it; what it looks like, what it feels like to hold,” said the boy’s attorney, Jeremy Elges. “It turned out there was a round in the chamber. He wasn’t savvy enough with guns to know that’s a risk.”

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Gun safety advocates have urged a public health approach that includes more research by the federal government, more public awareness and stricter state laws. But the problem of lax gun storage is difficult to address in part because it’s hard to quantify, and the federal government stopped trying 12 years ago.

The CDC had asked Americans about whether they kept loaded, unlocked guns in their homes as part of an annual telephone survey that asked 400,000 adults about various health risks. Using that data, researchers estimated that 1.5 million children lived in such homes — and the results varied greatly by state based on gun ownership levels.

But the CDC scrapped that line of questioning in 2004. State public health officials voted this year not to reintroduce the questions in next year’s survey, in part because of the political sensitivity around asking about gun ownership, which some see as an invasion of privacy.

“Some states did not think they would be allowed to ask such questions by their governor,” said Donald Shepherd, the survey coordinator for Iowa.

Gun control advocates say Child Access Prevention laws — on the books in about two dozen states — act as a deterrent. In general, those laws allow prosecutors to charge parents when children obtain their guns and use them to harm themselves or others.

Supporters say a study of a Florida law suggests they can save lives if implemented correctly. And the concept enjoys wide public approval: Two-thirds of Americans support laws making adults criminally liable for gun safety lapses that endanger children, according to an AP-GfK Poll in July.

But efforts to expand those laws have stalled in the face of opposition from the NRA and other gun rights supporters. Bills in several states, including Missouri and Tennessee, to create similar laws were introduced this year and died without action.

The NRA argues that more education and training should be the solution, not more laws and prosecutions. The NRA takes credit for improving safety through its Eddie Eagle Gun Safe program, which warns children not to touch any firearms they come across and to tell adults.

Critics dispute the effectiveness of that message, pointing to academic research that shows curious children will pick up loose guns, even when told otherwise.

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Bryson Mees-Hernandez’s grandfather had bought the gun and another firearm from his own father the year before, in California. The boy’s grandmother kept them under her side of the bed because she was often home alone, struggled with mobility due to back problems and was concerned for her safety after reading about local break-ins on Facebook.

The guns were kept in a case that had been used to transport them on an airplane back to Texas, and Anna Sperber believed that would keep them away from children. But Bryson was able to open the side of the case, reach in and grab the gun.

“I thought it was secure, but I was wrong. My grandbaby’s gone. And it happened while I was watching him and I failed him,” Sperber said, through tears. “I don’t want anyone to ever, ever go through this. It’s so horrible.”

A grand jury declined to bring charges against Sperber, but Crystal Mees blames her mother for the boy’s death and no longer talks to her or brings her two children around; she says she had warned her to keep the gun out of children’s reach. She plans to push for a “Bryson’s law” in Texas to make it easier for prosecutors to bring charges against adults who allow children access to firearms.

Both women attended the funeral, where relatives dressed up as Bryson’s beloved Avengers, and his casket had an Ironman theme. He was buried with Legos, toy cars and his grandfather’s favorite cologne.