LOS ANGELES - Investigators are still trying to determine why a 16-year-old Saugus High School student allegedly opened fire in a campus quad Thursday, killing two classmates and injuring three others before turning the weapon on himself.

"We are chasing all the leads available;" Los Angeles County Sheriff Alex Villanueva said. "At this stage, we don't know the motive."

Officials are piecing together how the shooter got the weapon, a .45-caliber handgun, and whether he targeted specific individuals.

Here is what we know:

The attack

Just before the start of second period, authorities and witnesses say, the student pulled a .45-caliber pistol from his backpack and began shooting his schoolmates.

A 16-year-old girl and a 14-year-old boy died at a hospital soon after. Three other students also were taken to nearby hospitals, one of whom was later released. Suspect Nathaniel Berhow, who authorities said ended the 16-second spasm of violence with a bullet to his head, was listed in critical condition.

A school surveillance camera recorded the violence, investigators said, but it was unclear whether the gunman targeted specific victims or fired at random.

The gunman apparently knew how many shots he had fired and reserved the final bullet for himself, Villanueva said. It was all over too quickly for anyone to intervene, although law enforcement was on the scene within minutes.

It was unclear how long Berhow had been on campus before the shooting. At the time, the quad was filled with students walking between classes.

The shooter

Authorities say Berhow launched the attack on his 16th birthday.

Friends and neighbors were stunned, saying he showed no signs of aggression. He ran junior varsity cross-country and helped younger members in his Boy Scout troop.

"He would have fun with the team and was a good kid," 10th grader Aidan Soto said. "The younger Scouts really looked up to him. He was there when they needed him with anything. I'm bewildered and looking for answers."

His family life in Santa Clarita was upended by his father's sudden death in December 2017, acquaintances said.

"He would tell me that he missed his father and that he loved him," said neighbor Jared Axen, 33.

The teen's father, Mark Berhow, was arrested for driving under the influence in 2013 and 2015 and pleaded no contest twice. The second time he was sentenced to 45 days in jail and five years' probation. According to jail records, he was also booked in 2015 on suspicion of attempted battery of a spouse. The Los Angeles County district attorney's office declined to file charges in that case, citing insufficient evidence.

A judge granted physical custody of the boy to his mother in August 2016, even though both parents still appeared to live in the family's small ranch home on Sycamore Creek Drive.

Mark Berhow was an avid hunter, authorities said, and investigators are trying to determine whether that is related to how his son allegedly got the weapon used in the attack.