Such guns are unregulated, easy to put together and almost impossible to trace because they have no serial number. The parts can be easily purchased online, without requiring a background check.
On November 14, Nathan Berhow killed two students and wounded three others at Saugus High School in Santa Clarita, near Los Angeles, California, Presstv Reported.
After the rampage, the 16-year-old fatally shot himself in the head, all using a weapon he'd assembled at home. Authorities do not know how Berhow obtained the gun.
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.
Despite this, the administration of US President Donald Trump has opposed any new gun control restrictions, partly due to pressure from the country’s powerful gun lobby.
Ghost guns are not illegal in the United States and should not 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, semi-automatic rifles 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.
Frames made with 3D printers can be adapted to take the parts of an AR-15 while also dodging the law.
"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," David Pucino, with the Giffords Law Center to Prevent Violence, told the AFP.
"You can purchase all those other parts and they're not considered a firearm," 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.
Hundreds of thousands of these are made in the US, but there is no statistic on how many because there is no registry of them.
Nearly 40,000 people were killed by firearms in 2017, which included around 2,500 school-aged children, according to the study released in September by the Joint Economic Committee in the US Congress.