LOS ANGELES (Reuters) - A 16-year-old boy who opened fire at his Southern California high school, killing two classmates and wounding three others before shooting himself in the head, used a “ghost gun” built from parts, the local sheriff said on Thursday.

Nathaniel Tennosuke Berhow pulled the .45-caliber semi-automatic pistol from his backpack on Nov. 14, his birthday, and shot students at Saugus High School in the Los Angeles suburb of Santa Clarita seemingly at random before turning the gun on himself. He died of his wounds the next day.

“When we did a search of the house, we encountered what’s called a kit gun. The weapon used in the homicide was also a kit gun,” Los Angeles County Sheriff Alex Villanueva told local KABC-TV in an interview.

“It was assembled from parts, had no serial number. So it becomes what’s known as a ‘ghost gun,’” Villanueva said.

The sheriff told KABC it was not yet clear if the teenage gunman put the weapon together himself. The boy’s father, who died in 2017, was an avid hunter who once owned six guns. But those weapons had been confiscated from him and destroyed, Villanueva said.

Kit guns are sold legally at gun shows and online and are sometimes called “80% percent guns” because they are purchased roughly 80% assembled, he said.

STILL NO MOTIVE

The sheriff told KABC that despite conducting 45 interviews and conducting an exhaustive search of Berhow’s home, police had not yet determined a motive in the rampage, the latest mass shooting at a U.S. school during a national debate over gun rights.

“All of those witnesses have been interviewed, we know the mechanism, the how, but the why still remains an elusive goal,” he said.

Villanueva said Berhow’s mother packed corn dogs, grapes and homemade cookies into his backpack before school that morning, evidence that “even the mother apparently was unaware of what was going to transpire.”

The two slain students were a 16-year-old girl and 14-year-old boy. Two other girls, aged 14 and 15, and a 14-year-old boy were wounded.

Federal agencies were working to unlock Berhow’s cellphone, which could contain clues to his plans, Villanueva said.

The scene at Saugus High School was reminiscent of other mass shootings at U.S. schools, including Marjory Stoneman Douglas High School in Parkland, Florida, where a former student with an assault rifle killed 17 people on Feb. 14, 2018.

It was the 85th incident of gunfire at a U.S. school this year, according to Everytown, a gun control advocacy group.