(CNN) The 16-year-old who killed two students and injured three others in a shooting at a California high school last week used a "ghost gun," Los Angeles County Sheriff Alex Villanueva told CNN affiliate KABC.

Villanueva said the firearm was a "kit gun," meaning it was assembled from separately acquired parts and had no serial number, making it untraceable to authorities.

"It becomes what's known as a ghost gun," he said.

These untraceable weapons can be assembled from kits bought online or at gun shows, Villanueva said, with the gun just partially assembled.

This allows buyers to sidestep the typical requirements that come with registering a firearm, including background checks, the Los Angeles Police Department and the ATF said in a news release last year