The 16-year-old who opened fire Thursday at a high school in Santa Clarita, Calif., killing two people and wounding three others, has died of a self-inflicted gunshot wound to the head, authorities said.

Nathaniel Tennosuke Berhow shot himself on his 16th birthday after pulling a gun from his backpack at Saugus High School, where he was a junior, the Los Angeles County Sheriff’s Department said. Berhow received treatment, but died Friday at about 3:30 p.m. local time with his mother present, according to the department.

Berhow’s motive in the attack remains unclear, sheriff’s department spokesman Edmo Luna said Friday evening, a day after Saugus High students evacuated in the first fatal shooting this academic year on U.S. school grounds. County Sheriff Alex Villanueva said that the gunman shot seemingly at random but appeared “very familiar with firing the weapon” and did not commit a “spur-of-the-moment act."

It was at least the seventh shooting of the school year, according to a Washington Post analysis — a familiar tragedy as data shows more than 230,000 children have experienced gun violence at their schools since 1999′s massacre at Columbine High.

Deputies were on the scene two minutes after a 911 call reported the Saugus High shooting at 7:38 a.m. Thursday, authorities said. They found the shooter, dressed in black, among six students with gunshot wounds in the school’s outdoor quad area. A seventh person received treatment for noncritical injuries, according to officials.

The sheriff’s office has identified one of Berhow’s classmates killed Thursday as 15-year-old Gracie Muehlberger. A coroner identified the other as 14-year-old Dominic Blackwell. One student hospitalized has been released, while two others are not expected to leave until this weekend.

Authorities said they found a .45-caliber semiautomatic handgun on the scene and were still working to trace the gun Friday, with help from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

Officials said Friday that they have interviewed more than 40 people but have not found writing from Berhow that could shed light on his thinking before the attack.

As they continue to investigate, campuses across the William S. Hart Union High School District were closed Friday in a show of respect to victims.

Students at Saugus High did everything they were taught to in an active shooter training held just weeks before Thursday’s “senseless violence,” Villanueva told reporters. They barricaded doors, covered windows and sheltered in place, he said, in a “job well done" that he hopes they never have to repeat.

“It’s a tough day here in Santa Clarita,” the sheriff said, urging people to hold their kids “a little tighter.”

Mayor Marsha McLean echoed Villanueva, saying the shooting has rattled a close-knit community.

“As I speak to our residents, we are all feeling what happened,” she said.