Sometimes the motives of mass shooters are enigmatic, but not in this case. Even though there is still more to learn, there is apparently little mystery about the most recent mass shooting. The 19-year-old accused of the massacre at Marjory Stoneman Douglas High School in Parkland, Fla., on Wednesday — Valentine’s Day — should never, as a matter of law and common sense, have had access to a gun, much less the AR-15 assault rifle he apparently obtained legally and owned for some time, or the large-capacity magazines police say he used to kill 17 and wound 16.

We already know, and authorities already knew before the shooting, that he was a disgruntled teenager with a history of disturbing and disruptive behavior that set off alarm bells in those who encountered him. In comments about the suspect, acquaintances, schoolmates and various officials all concur that he had “always been a troubled kid” who was unusually fascinated with guns and knives. Broward County Sheriff Scott Israel said that some of what has been uncovered about him is “very, very disturbing.” Little wonder that he was expelled from the high school for disciplinary reasons. Last year, an email to the school’s faculty cautioned that he should not be let onto school grounds with a backpack. One student who knew him said that “he seemed like the kind of kid who would do something like this.”

So what difference, if any, could public policy have made to avert the outcome that everyone appeared to fear? Quite a bit, actually.

Assault weapons were restricted for 10 years by federal law — a law that was allowed to lapse in 2004. Included in that legislation was a restriction on large-capacity bullet magazines: those holding more than 10 rounds. (Today, seven states retain similar restrictions.)

The absence of comprehensive record-keeping and the apparent ease by which a teenager obtained and displayed firearms long before the shooting also suggest lapses in existing policy.

Our own past suggests that these are not new problems.

On another St. Valentine’s day 89 years ago, seven Chicago gangsters in the employ of Bugs Moran were murdered by four members of a rival gang led by Al Capone. The four, dressed as police officers, lined up the seven against a wall and executed them. The resultant public outrage, riding a crescendo of national dismay over rising criminal violence, helped fuel growing public pressure for gun laws to stem the use of what were then called “gangster weapons” in crime. From 1927 to 1934, at least 27 states enacted laws to bar fully automatic weapons such as the notorious Tommy gun, and other gangster weapons such as sawed-off shotguns. During the same period, between seven and 10 states enacted laws that severely restricted or even barred semiautomatic weapons — weapons that fired in the same manner as AR-type weapons of today.

Moreover, concern about the ability to fire numerous rounds without reloading, whether by fully automatic or semiautomatic means, also led to restrictions nine decades ago. In the late 1920s and early 1930s, no fewer than 17 states enacted various restrictions on the number of rounds that could be fired without reloading.

These growing public concerns finally led to the enactment of the first comprehensive national gun law, the National Firearms Act of 1934. In other words, robust state and federal regulations resulted from the crime epidemic of the 1920s and early 1930s. Yet in the past three decades, we’ve seen mostly a reversal of that trend.

Although crime, including violent crime, has progressively declined in the United States since the early 1990s, mass shootings have increased. Even as they amount to about 2 percent of all gun fatalities annually, they are part of a distinct and alarming trend. Since the Sandy Hook Elementary School shooting in Newtown, Conn., in 2012, more than 1,600 mass shootings have occurred, and the five worst mass shootings in our history have taken place in the past decade. Two of those five happened just last year, when there were a total of 346 mass shootings, according to Gun Violence Archive. Assault weapons constitute about 2 percent of all firearms in the United States, but such weapons have been used in 27 percent of public mass shootings from 1999 to 2013.

All of this calls, at the least, for a reexamination of our laws and incomplete record-keeping practices, especially at the intersection of firearms and teenagers. (States enacted laws restricting access to guns by minors as early as the late 1800s.)

The history of gun laws in the United States tells us that gun regulations are by no means a new idea, and they haven’t always been considered antithetical to the Constitution. Yet contemporary gun politics has at least partly succeeded in turning the debate over gun laws into a kind of third rail. But that is an artifact of contemporary hyperpolarized politics. This would be a good time to push aside the hype and learn the lessons from our past. Or else we’ll keep repeating recent history, instead.

Post opinions writers Molly Roberts, Karen Tumulty, Christine Emba and E.J. Dionne discuss the political spin surrounding the Parkland, Florida school shooting. (The Washington Post)

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