NEW YORK (NYTIMES) - The 16-year-old student who killed two of his classmates in Santa Clarita, California, died from his self-inflicted gunshot wound on Friday (Nov 15), complicating investigators' efforts to understand why he pulled a handgun from his backpack and shot five other people.

Police said they know that his mother dropped him off at school on Thursday morning. They know there were six more weapons inside his house. What they do not know is why he did it.

"We did not find any manifesto, any diary that spelled it out, any suicide note or any writings which will clearly identify his motive behind this assault," Captain Kent Wegener of the Los Angeles County Sheriff's Department said at a news conference on Friday before the gunman's death. He said that police had conducted more than 40 interviews, but that "no motive or rationale" had been uncovered.

Authorities identified the gunman as Nathaniel T. Berhow. Although he shot himself in the head, he initially survived and was transported to a hospital along with the other victims.

Sheriff Alex Villanueva of Los Angeles County said the gunman was silent as he shot the five students at Saugus High School. He seemed to have planned the attack, the sheriff said, and also seemed familiar with how to use the gun, quickly fixing a malfunction and firing all but one round before shooting himself in the head. The shooting was captured on video, and the gunman appeared to have selected his victims at random.

"It doesn't appear he had any interaction with anyone," Mr Villanueva said. "He was just standing by himself, at one point walked to the centre of that quad area - dropped the backpack - withdrew a firearm and just started firing."

The coroner's office identified the two students who died in the shooting as Gracie Anne Muehlberger, a 15-year-old girl, and Dominic Blackwell, a 14-year-old boy. Gracie's father said in a brief phone interview on Thursday afternoon that the family was grieving.

There is no indication that Berhow's mother knew he had a gun, Mr Villanueva said. Investigators are tracing the weapon as well as six other guns they found at the gunman's home, some of which were not registered.

Even as police said they did not know what inspired the gunman, they said it was clear he had planned the attack.

"It wasn't a spur-of-the-moment act," Mr Villanueva said.

When students realised the popping noises they heard were gunshots, they fled into classrooms and away from the school. Dozens took refuge in a choir classroom, which a teacher barricaded with a grand piano before tending to a student's gunshot wounds.

The injured victims - a 14-year-old girl, a 15-year-old girl and a 14-year-old boy - were all in stable condition by Thursday night, authorities said.

The FBI was scouring social media for clues, but police said on Friday that an Instagram account they initially believed was the gunman's was actually not his.

Santa Clarita is home to many police officers, firefighters and emergency workers, and the first three police officers to respond to the shooting were off-duty officers who had dropped children off at the school.

Detective Daniel Finn had just dropped his girlfriend's son off at the school when he saw hundreds of children running from gunfire.

Mr Finn, who works in the Santa Clarita Valley Sheriff's Station, turned his car around, ran onto campus and provided medical aid to several victims, whom he found lying on the ground.

He was quickly joined by an off-duty officer from the Inglewood Police Department and another from the Los Angeles Police Department.

Deputy James Callahan, a school resource officer, was on the scene seconds later, and a flood of additional officers soon arrived.

"When you're a school resource deputy, you take a lot of pride in keeping your campus secure," he said on Friday. "You never think a tragic thing like this is going to happen."

Police officers saw a gun on the ground in the quad but were not sure that the gunman had shot himself until they reviewed the surveillance video and confirmed that he was one of the patients who had been transported to a hospital.

As officers searched the campus, students cowered in classrooms. Hundreds of students fled to a nearby park, where they reunited in tearful hugs with their parents, many of whom had raced to the school after getting texts from their children.