What stands out about the mass shooting at Marysville-Pilchuck High School on Friday is the sheer mystery of it.

School shootings are all too familiar in this country. Dozens have occurred since the infamous Columbine massacre outside Denver 15 years ago. But almost always, there’s been at least a hint of an explanation. Mental illness, social isolation, bullying, revenge – something.

So far, nothing begins to explain why the 15-year-old Jaylen Fryberg gathered five of his friends to a table in the school cafeteria, pulled out a handgun and shot them all, then himself. Three of the six, including Fryberg, are now dead, and two remain in critical condition.

News organizations tried to ferret out a racial angle. Fryberg was of Snohomish heritage; he belonged to the Tulalip Tribes, a group of seven Salish tribes occupying a reservation on Puget Sound just west of Marysville. But Fryberg’s victims included fellow tribal members; two were close relatives. This was something akin to domestic violence.

In any case, Tulalip students are remarkably well integrated in the Marysville school system compared to Indian students in many other communities. The Tulalips themselves are prosperous and respected; one of their members represents the area as a state senator.

Mental illness? Aside from a recent fight and spate of distressed Twitter messages – neither of them exceptional for a boy his age – no one has so far reported bizarre behavior on his part prior to the shootings.

Was Fryberg a misfit living on the margins? Quite the opposite.

By all accounts, he was outgoing and popular. He’d recently been elected “homecoming prince” of his freshman class. He was a good football player.

He belonged to a prominent Tulalip family; some of his elders are influential tribal leaders. His mother was a member of the Marysville School Board.

He seems to have been especially close to his father, with whom he hunted and fished. He was connected to his Salish roots; he sang, danced and drummed in tribal ceremonies.

After the shooting, one of Fryberg’s fellow Marysville students said, “People loved him. He was very respectful.” Another described him as “… just a really nice kid and all-around good person.”

A tribal leader described him as “a huggy guy” who “loved his friends and family and his little cousins.” Andrew Gobin, a fellow Tulalip and a reporter for the Everett Herald, said, “He seemed to have it all.”

In other words, Fryberg doesn’t fit the profile of someone who would strike back at the world with lethal force. He fits the profile of someone who wouldn’t do it.

When a school shooting happens, the debate quickly turns to the regulation of firearms. That’s an important question, but it has no obvious bearing on this tragedy.

Fryberg reportedly got the handgun from an unspecified relative, who appears to have purchased it legally. It wasn’t an “assault weapon,” so it wouldn’t have been subject to restrictions on rifles.

Initiative 594, now on the ballot, wouldn’t have prevented him from getting the gun. In fact, as a federally recognized Indian nation, the Tulalips may not be subject to most state firearm regulations. It was already illegal to bring the gun into the high school – but someone who is willing to kill himself won’t be deterred by rules.

To prevent a horror like this, we have to anticipate it. To anticipate it, we have to understand it or at least recognize warning signs. So far, we’ve got nothing – no understanding, no warning signs, no explanation. Just a bewildering mystery.