Investigators in southern California are working to figure out why a suspect opened fire outside his high school on his 16th birthday, killing two fellow students and wounding more before turning the gun on himself.

The latest deadly school shooting to hit the US started early on Thursday morning at Saugus high school in Santa Clarita, 30 miles north of Los Angeles.

The student pulled out a .45-caliber handgun around 7.30am in a large central outdoor area at the school, shooting five classmates and then himself, authorities said. A 16-year-old girl and a 14-year-old boy succumbed to their injuries. They were identified as Gracie Anne Muehlberger and Dominic Blackwell.

The suspect was admitted to the Henry Mayo Newhall hospital in critical condition, and died on Friday afternoon.

After more than 40 interviews and evaluation of evidence, no motive had been established, said Capt. Kent Wegener, of the Los Angeles County Sheriff’s Department’s homicide unit. He said no manifesto, diary or suicide note had been found.

“It still remains a mystery why,” said Alex Villanueva, the LA county sheriff, at a news conference.

The teen stood by himself, did not appear to interact with anyone and then walked to the center of the quad, Villanueva said.

“As far as we know the actual targets were at random,” the sheriff said.

Villanueva said the conclusion that the attack was planned was based on the shooter bringing the weapon, handling it with enough expertise and counting the rounds fired.

“It wasn’t a spur-of-the-moment act,” he said.

The origin of the gun was being investigated.

Doctors said Friday morning that two girls, ages 14 and 15, who were both shot in the torso, were doing well and should be released from the hospital over the weekend. A 14-year-old boy was treated and released from another hospital, authorities said. The names of the injured students were not released.

Facebook Twitter Pinterest People hug each other during a vigil for the shooting victims in Santa Clarita. Photograph: Hans Gutknecht/AP

Shauna Orandi, 16, told the Guardian on Thursday she was in her Spanish class when she heard four gunshots that she initially mistook for musical instruments. A student who burst into the room said he had seen the gunman and classmates were stunned into silence, she said.

“My worst nightmare actually came true,” she said. “This is it. I’m gonna die,” she recalled thinking.

Rosie Rodriguez, a freshman, was walking up the library stairs when she heard noises that she said “sounded like balloons” popping. She realized they were gunshots when she saw other students running.

She ran across the street to a home, where a person she didn’t know gave shelter to her and about 10 other students.

“I just heard a lot of kids crying. We were scared,” Rodriguez said.

People who knew the suspect described him as a quiet, smart kid who they would never have expected would violently snap. A nextdoor neighbor said he kept to himself but was never threatening. “You wouldn’t expect anything like that from him,” said Ryan McCracken.

Police continue to search for clues. On Thursday evening, authorities revealed that the bio on an Instagram account believed to have belonged to the suspect, and reviewed after the shooting, read: “Saugus, have fun at school tomorrow.” The bio has since been changed, but it was unclear when or by whom.

‘When will we come together?’

In a Thursday afternoon press conference that felt painfully familiar, local leaders mourned another senseless tragedy amid a continuing scourge of school shootings. They pledged that Santa Clarita would band together to recover.

“This is a very tragic day for our community,” the local assembly member Christy Smith told reporters, thanking law enforcement, who responded to the incident within minutes. “We are very close-knit. We will come together and survive but we can’t afford another day like this.”

“Across the nation I frequently hear ‘no more, no more’,” added Robert Lewis, the captain of the Santa Clarita Valley sheriff’s station. “When are we as Santa Clarita and other communities going to come together?”

Facebook Twitter Pinterest First responders at Saugus high school, where at least two students were killed on Thursday. Photograph: Christian Monterrosa/AP

At a family reunification center at a park next to the school, some students sat at picnic benches inside a community garden and talked to law enforcement as loud helicopters hovered above. Parents held their children as they walked to their cars.

April Dooley, a former choir teacher at Saugus high, said the school had extensively prepared for this kind of tragedy, but she still never imagined it would happen in her own community.

“I’m in total shock. It’s unbelievable. Saugus high is such a quiet and sweet community – extremely benevolent,” said Dooley, 60, who retired in December. She heard that one wounded victim ended up in the choir room where she used to teach, and said she felt a lot of pain for the teacher who replaced her, who was in her first year on the job: “She’s still figuring out how to be a teacher, and now she’s figuring out how to deal with an active shooter. How can one do that? It seems impossible for anyone … I’m so proud of her.”

Dooley said that, as a teacher, she often thought about what she would do if a shooter threatened her class: “We are their defense. Our job is to keep them safe … We are in a choir room, there are things to defend ourselves, a piano we can hide behind. We were all very prepared.”

Facebook Twitter Pinterest Flowers are dropped off at a makeshift memorial not far from the school. Photograph: Frederic J Brown/AFP via Getty Images

Dooley said she drove over as soon as she heard the news to offer any support she could: “I’m sad I wasn’t there to take care of my babies.” She added that she wanted to see the government take action on background checks.

Scott Wilk, a state senator who lives a mile away from the school, said he was still processing the news. His children previously attended Saugus high, and he learned this morning that a friend’s daughter had been shot but was expected to survive.

“It is just indescribable. This has got to stop,” he said, noting that it felt as if the Thousand Oaks mass shooting, which killed 12 in the region last year, had been quickly forgotten: “It seemed like that was a one-day story, and it was gone. We have become so desensitized to this, like it’s the new normal. It can’t be the new normal.”

Wilk said he wanted to see stronger investments in mental health resources for youth: “Our most precious resources are our children.” He noted that when he recently visited a fourth-grade class, one of the students talked to him about wanting better security.

Condolences poured in from across the country, with many Democratic lawmakers pointing to the tragedy to once again argue US lawmakers should pass stricter gun control legislation.

The California senator and presidential candidate Kamala Harris said she was heartbroken and was praying for Santa Clarita. “Our children and communities are being terrorized. We can’t accept this,” she said.

Facebook Twitter Pinterest Students reunite with their families after the shooting at Saugus high school. Photograph: Sarah Reingewirtz/AP

“Another sad example of a school shooting & why students across the country live in fear of gun violence,” wrote the Minnesota senator Amy Klobuchar, also a presidential hopeful.

“I’m heartsick for the victims of this horrifying shooting and their families,” said the Massachusetts senator and presidential candidate Elizabeth Warren.

Back at the reunification center, Kris Hough, a field representative in Wilk’s office who had showed up to help, said she wanted more than “thoughts and prayers”.

“I don’t want to say ‘thoughts and prayers’ or any of that, because that’s not what we need,” she said. “We need this to stop. How many more of these do we have to see before it stops?”

Hough noted that it was such a small community that many law enforcement officials tasked with responding to the tragedy had loved ones at the school: “We all think every time one of these things happen that it’s not going to happen in my safe little bubble here. It’s scary to know it can happen to you just as easy as anyone else.”

She added, “We have all sat in front of our TVs watching in horror as these things happen. Now it’s our turn.”

Agencies contributed reporting