LOS ANGELES — The 16-year-old boy who fatally shot two fellow students and wounded three others last week at a Southern California high school used an unregistered, untraceable “ghost gun,” Los Angeles County Sheriff Alex Villanueva said Thursday.

Villanueva told media outlets that Nathaniel Berhow’s .45 caliber, 1911-model replica semi-automatic pistol was assembled from gun parts and did not have a serial number.

Such weapons are a growing problem for law enforcement around the country because the parts are easy to obtain and the guns take limited expertise to build. In Southern California, federal authorities say one-third of all the firearms seized are ghost guns.

California has among the strictest gun laws in the country, but they are based on traditional firearms that are made by manufacturers and labeled so ownership can be traced.

“Congress and state legislatures enact all these crimes about gun registration but now the gun industry is creating a way to just bypass the entire thing by creating a mechanism to manufacture weapons yourself,” Villanueva said.