After a 9-year-old girl in Arizona accidentally shot and killed her shooting range instructor with an Uzi last week, it raised what would appear to be a fairly obvious question: How often do children in the United States — where unintentional or accidental shootings occur with some frequency — fatally shoot people by accident?

The answer: We don’t actually know for sure. At least that’s what the people and agencies tracking this topic say.

“We know how many times children die each year as a result of gun deaths,” Jon S. Vernick, co-director of the Johns Hopkins Center for Gun Policy and Research, said in an interview last week. “We don’t know how many times children pull the trigger and someone dies.”

Vernick said the data is out there, but it has not been pulled together or compiled by anyone.

Agencies that compile statistics regarding shooting deaths told The Post that while they have data on many aspects of shooting deaths, this figure was unavailable. The Centers for Disease Control and Prevention said that there is no nationwide data regarding the age of the person who pulls the trigger in an unintentional shooting. The Justice Department offered a similar response. “We do not have any statistics available regarding this topic,” a statistician with the Bureau of Justice Statistics wrote in an e-mail last week.

Some details regarding gun deaths are known, collected and reported. We know how many gun deaths occur in a given year, which makes sense, because when people are shot and killed, there are death certificates and there are reports by medical examiners, and national reports can conclusively come up with a number. (There were 32,351 such deaths in 2011, according to the CDC.) We know how many gun deaths were declared accidental (591 in 2011, the CDC says). And we know that 102 people killed in these accidental gun deaths in 2011 were younger than 18, according to Vernick, with half of these children younger than age 13.

But when you try to look into how many of the people pulling the trigger in accidental gun deaths were also children, you run into a problem.

When children unintentionally shoot themselves or other people, media reports typically follow. A three-year-old boy is playing with a gun and shoots himself in the face. A four-year-old girl discovers a gun and shoots her four-year-old cousin, killing him. A three-year-old boy shoots himself in the head. A five-year-old accidentally shoots a three-year-old girl. A five-year-old boy accidentally shoots and kills himself. A four-year-old boy accidentally shoots himself. A two-year-old boy shoots and kills his 11-year-old sister. It goes on like this, story after story of unintentional shootings involving children that lead to injuries or deaths. (Many unintentional shootings of children occur when they are with people of similar ages, Vernick said, though many also involve children by themselves.)

The National Violent Death Reporting System (NVDRS), which the CDC launched in 2002, does combine data from death certificates, medical examiner reports and law enforcement reports to try to produce this type of information. However, this system currently only operates in 18 states, so the numbers it offers are not national and the CDC cautions that the data should not be viewed as nationally representative. Still, it offers some information: Across the 17 states the NVDRS has data for from 2011, there were 11 unintentional firearm deaths that year in which the person pulling the trigger was age 14 or younger.

In addition to uncertainty regarding how many children accidentally shoot and kill people, the overall number of accidental gun deaths may also be incomplete. The CDC’s numbers, available through the National Center for Health Statistics, are collected from a mortality database that includes causes of death as determined by medical examiners, coroners and attending physicians. Yet this, again, is not foolproof. Medical examiners may say that a shooting death that appears to be unintentional was a homicide or say the cause cannot be determined, which is a separate category.

Vernick offered an example of what would appear to be an unintentional shooting: A teenager is playing with a gun that he thinks is empty pulls the trigger, shooting and killing another teenager. In this case, such a death could actually be deemed a homicide, since the teen intentionally pulled the trigger, even if there was no intent to actually fire a bullet.

“We know with precision the number of gun deaths,” Vernick said. “What we don’t know is all of the characteristics of those deaths that we’d like to know.”

An investigation carried out by the New York Times last year found significantly more accidental killings involving children (age 14 and younger) than had been reported in eight states. That investigation showed that medical examiners could issue wildly inconsistent rulings, even deeming accidental self-inflicted shootings to be homicides.

“I try to tell people when they look at the accidental data, particularly for children, you have to recognize it’s an underestimate,” Bob Anderson, head of the mortality statistics branch at the CDC’s National Center for Health Statistics, told the Times.

Dan Gross, president of the Brady Campaign to Prevent Gun Violence, said that nine children are unintentionally shot each day in the U.S. He said that the Arizona accident, while “a terrible tragedy,” was also an outlier, because it involved an Uzi and a shooting range.

“This is not a problem that leads to thousands of deaths across our country every year,” Gross said. “The bigger problem of kids having unsafe access to guns is.”

Gun advocates said in interviews after the Uzi incident that there are benefits to teaching young children to shoot certain guns. However, some still questioned allowing such a small child handle a gun as powerful as the Uzi, which can fire hundreds of bullets at a time. “I just don’t think a kid has any business with a weapon like that,” said Nancy Lichtman, 43, who views shooting guns as a wholesome family activity. Her adult daughter and teenage son both shoot regularly.

Meanwhile, while federal law states that people younger than 18 cannot possess a handgun (albeit with exceptions including target practice), there is no federal minimum age for possessing a long gun like a shotgun or a rifle, according to the Law Center to Prevent Gun Violence. And most states do not set a minimum age for possessing a long gun; 30 states do not have a minimum age for this, including Arizona, where the shooting range instructor was killed.

The fact that we don’t know the number of times a child has accidentally shot and killed someone is an odd gray area, particularly given how even the staunchest guns advocates seem to agree about the importance of storing guns away from children. The National Rifle Association says on its Web site that it is a responsibility of gun owners to store guns away from children, adding that parents with a gun must “absolutely ensure that it is inaccessible to a child.” (The NRA did not respond to a request for comment after the shooting death in Arizona.)

Researchers praise having access to more data, which can help show trends over time, guide decisions made on a local level or depict any movement after laws or policies are enacted. “To stop violent deaths, we must first understand all the facts,” the NVDRS site says. And there is also the issue of simply not knowing how many children may be impacted by being involved in an accidental shooting.

“There are two victims in this tragedy,” Shannon Watts, founder of the gun-control group Moms Demand Action for Gun Sense in America, said in an interview after the Arizona shooting range death. “There’s the instructor, but there’s also this poor nine-year-old girl who has to live the rest of her life knowing that because of an adult’s mistake, she has killed someone accidentally.”

Sandhya Somashekhar contributed to this post.