SANTA CLARITA, Calif. — A Saugus High School student on Thursday walked into the campus quad, pulled a handgun from his backpack and shot five classmates, killing a 16-year-old girl and a 14-year-old boy and then fired a bullet into his own head, leaving himself in grave condition, according to officials. 

Authorities said the gunman opened fire on his birthday. 

Sheriff's Capt. Kent Wegener said detectives reviewed security video that shows the gunman reach into his backback, pull out a .45-caliber semiautomatic handgun and open fire. The weapon was recovered at the scene. 

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The attack lasted about 16 seconds, and the attacker apparently fired at random because he had no known connection to the victims, sheriff's homicide Capt. Kent Wegener said.

"It's a sad day in Saugus, it's a sad day in Los Angeles County and the nation for another tragic shooting at a school," said Los Angeles County Sheriff Alex Villanueva in a late-morning news conference.

The shooting happened around 7:30 a.m. in school's quad area at the campus at 21900 Centurion Way, according to the Los Angeles County Sheriff's Department. 

Also wounded were a 14-year-old boy, a 14-year-old girl and a 15-year- old girl. One other person was taken to Olive View-UCLA Medical Center with a minor, non-gunshot injury, county Fire Chief Daryl Osby said.

The two injured girls were being treated at Providence Holy Cross Medical Center in Mission Hills.

A surgeon at the hospital told media one of the girls was shot in a shoulder and would not require surgery. Another girl at the hospital had a gunshot wound just above the belly button, with the bullet lodging near her hip. She was expected to undergo surgery later Thursday.

A spokesman for Henry Mayo Hospital said one person who was brought to the hospital in good condition -- believed to be the 14-year-old boy -- was treated and released early Thursday afternoon. Henry Mayo officials confirmed earlier that a total of four patients had been brought to the hospital -- two have died and one now released.

The gunman remains at a Santa Clarita in critical condition, and because that information has now become public, Sheriff Villanueva said they would have to increase their security around a hospital.

After learning the suspect's name by interviewing eyewitnesses and reviewing surveillance video, deputies went to his family's home and conducted an initial search to ensure there were no additional victims there, but none were found. Villanueva later said the teen's mother and girlfriend were being interviewed by detectives.

The student was not immediately identified. Neighbors told reporters the family was mostly quiet, noting that the teen's father had died two years ago, possibly from a heart attack.

According to investigators, a post on what is believed to be the gunman’s Instagram account said: “Saugus, have fun at school tomorrow.”

That post was later removed and the account was active after the shooting and the gunman was hospitalized, according to investigators - leading them to believe someone else besides the gunman has access to the account.

Investigators do not know if the gunman gave someone his Instagram account password or if the account had been hacked.

Below is the initial police scanner audio of when authorities responded to the school shooting Thursday morning.

According to Villanueva, off-duty officers whose children went to Saugus High School were the first on scene and provided first aid to the victims. 

One off-duty officer had just dropped off a family member when he saw kids running from the gun fire. He then turned around and arrived within seconds. When the two other off-duty personnel arrived, they began to provide first aid, potentially saving the lives of  several students.

Villanueva said, "With a heavy heart, we're going to move forward with the investigation, figure out what went wrong."

"I hate to have Saugus be added to the names of Columbine, Parkland, Sandy Hook, but it's a reality that affects us all throughout the nation, something we're going to have to deal with," he said. "... We've got to figure out what are we doing wrong and how can we stop this from happening in the future, so we'll figure that out over time."

Paul Delacourt, assistant director in charge of the FBI's Los Angeles field office, also decried what is now the latest in a string of U.S. school shootings.

"... We're talking about something that no parents should ever have to go through, no high school students should ever have to worry about, yet we're here to talk about it again."

He noted that the FBI had sent crisis response resources, were on the scene in support of the Los Angeles County Sheriff's Department and were collaborating on interviews with victims and witnesses.

"We will run all the leads to (the) ground, but at this point we have no indication of any motivation or ideology."

"Once again, it's sad that we have had to bring those resources to bear in another situation like this."

The San Diego Police Department tweeted Thursday morning that the department "is aware of the tragic events in Santa Clarita," where a student opened fire Thursday morning at Saugus High School. "Though the incident appears to be an isolated one, our patrol divisions and juvenile service team officers will begin immediately providing an increased presence at all schools within our jurisdiction," according to the tweet. 

The FBI says there’s no reason to believe he was acting on behalf of a group or ideology or that there were any conspirators.

President Donald Trump wrote on his Twitter page Thursday afternoon, "We continue to monitor the terrible events at Saugus High School in Santa Clarita, California, through our ongoing communications with local, state and federal authorities. We send our deepest condolences to the families and friends of those tragically lost, and we pray for the speedy recovery of the wounded."

There was still no word on a motive in the shootings. 

A prayer vigil was at Grace Baptist Church, 22833 Copper Hill Drive, in Santa Clarita. Another community prayer service was held at St. Stephen's Episcopal Church, 24901 Orchard Village Road, Santa Clarita.

A community vigil is also being planned at 7 p.m. Sunday at Santa Clarita's Central Park.