After his mother dropped him off at school, Nathan Berhow pulled a .45-caliber pistol out of his backpack, opened fire and killed two classmates, all using a weapon he'd assembled at home.
Such guns are sometimes called "ghost guns" -- unregulated, easy to put together and almost impossible to trace because they have no serial number.
The parts are readily available online, with no need for a background check.
After opening fire at Saugus High School in Santa Clarita, near Los Angeles, Berhow shot himself fatally in the head. Three people were wounded in his rampage.
The authorities do not know how Berhow obtained the gun.
The boy's father, who died in 2017, liked hunting and owned six legally acquired guns, which were seized and destroyed after his death.
The United States has lived through a sharp rise in mass shootings in recent years, alarming the public and triggering new debate on how to control this epidemic of violence.
But the administration of President Donald Trump has opposed any new gun control restrictions.
There is nothing illegal about ghost guns, which are not to be confused with guns that have had their serial number erased.
They are also called "80 percent guns" because the parts are acquired separately and require a bare minimum of work to make them operational.
There are kits to make pistols and even AR-15s, the weapon used in several recent mass shootings.
They escape regulation because of one part, the only component that is really regulated in the US. For handguns, it's called the frame, and for rifles it's known as the receiver.
"That one piece is the component that is subject to a background check, and is the one that has to be serialized under federal law," said David Pucino, with the Giffords Law Center to Prevent Violence.
"You can purchase all those other parts and they're not considered a firearm," he added.
"What companies do is sell the frame, then, with a little bit of work left to do on it.
"And what they do is they leave a few holes left to be drilled in the frame or in the receiver. And they call this 80 percent completed."
"And if they had bought it with those holes drilled out, it would have been serialized" and required a background check, Pucino said.
Frames made with 3D printers can be adapted to take the parts of an AR-15 while also dodging the law.
In the US, hundreds of thousands of these are made but there is no statistic on how many because there is no registry of them.
The Bureau of Alcohol, Tobacco and Firearms says 30 percent of weapons found at crime scenes in California have no serial number and thus cannot be traced in a criminal probe.
New Jersey has banned the use of gun parts without serial numbers, while California has passed a law that will come into force in five years and bars anyone without a gun sales license from marketing them.
These steps will help limit the spread of ghost guns but the Giffords Center says only a national law on serial numbers can address the problem.
Mark Tallman, a professor in security studies at Colorado State University, says ghost weapons go back to the early days of the country and their existence does not have much effect on crime rates.
"Most of the commercial guns in the United States are unregistered, and most of America's gun crimes are undertaken by offenders who are already unable to pass background checks to buy them," said Tallman.
"Traffickers can also buy 'normal' guns and use inexpensive tools to destroy their serial numbers, making them almost as difficult to trace as a homemade 'ghost gun.' So, the overall impact of homemade guns on this complicated dynamic is still difficult to assess," he added.
Pucini said he fears that more ghost guns will be used in school shootings because sellers do not check the age of buyers.
But Tallman said the increase would not be due just to these weapons.
"In my opinion, it's not realistic to expect a huge reduction in shootings just because we crack down on homemade guns, because many shootings were already happening before the 'ghost gun' craze, and many shootings still involve 'normal' guns," he said.