Two students -- a 16-year-old girl and a 14-year-old boy -- were killed and three other teens were injured Thursday after a 16-year-old suspect -- who was transported to a hospital and was in "grave" condition -- opened fire at Saugus High School in California, officials said.

"Suspect is in custody and being treated at a local hospital," Los Angeles County Sheriff Alex Villanueva had tweeted.

Six people, all students, had gunshot wounds when police arrived at the scene around 7:40 a.m., Los Angeles County Sheriff's Capt. Kent Wegener said Thursday afternoon during a news conference. It was later discovered that one of the victims was the suspect, an Asian-American student at the school whose 16th birthday was Thursday, Wegener said.

The victims were a 16-year-old girl who died at 9:23 a.m., a 14-year-old girl, a 15-year-old girl and two 14-year-old boys, according to officials. One of the 14-year-old victims died in the hospital, Villanueva said at the news conference, noting that it was not the shooter.

There was also one non-critically injured victim who was transported to a hospital, for a total of seven victims including the suspect, investigators said.

As of 7 p.m., one victim was in stable condition and had been released from the hospital. Another victim was in critical condition.

The weapon -- a .45 semi-automatic pistol that had no remaining bullets -- was recovered at the scene, Wegener said.

"Detectives have reviewed video that clearly shows the suspect withdraw a handgun and wound five people and shoot himself in the head," he said.

The suspect's mother and girlfriend were at the police station and a search warrant was being written and will be conducted at the suspect's home for evidence, Wegener said.

No motive was immediately known. An investigation was begun to determine what happened and why, officials said, and the FBI was on the scene.

Sheriff Alex Villanueva said in a news conference that three off-duty officers who were near the school were able to reach the scene immediately, likely saving lives. One officer had just dropped off his child at school, and as he was pulling out he saw children running away from the suspect. He turned right around and entered the school within seconds. Uniformed officers followed minutes later.

Police said they were in the process of interviewing students who were witnesses.

Fellow student Ryan Payae told KCBS he reported the post as a social media threat to police, but Sheriff Villanueva said in a news conference there was nothing police could do about such a vague posting.

The shooter had been described as a "male Asian suspect" wearing black clothing who was last seen walking away from the scene, the sheriff's office had said.

The Henry Mayo Hospital said it received four patients from the incident.

"Female and two males are in critical condition. One male is in good condition," the hospital tweeted prior to the news conference.

Two victims were taken to Providence Holy Cross Health Center in Santa Clarita, according to Los Angeles County Fire Department Chief Daryl Osby.

"This is a pretty sobering incident for those involved, including first responders," he said during the news conference, noting that the first responder community had children who attended the high school.

Santa Clarita Mayor Marsha McLean said her grandaughter attended the high school, and that even though the city is the third-largest in Los Angeles County, "we are a close-knit family-oriented community, and our family sticks together."

"The feeling that went over me was indescribable," she said of the moment she heard the news. "I felt the same thing as our families."

"I want every single family member to know we are with you, our heart is with you, and that's all I can say as to this horrible thing," McLean added. "There are no words to describe it."

President Trump offered his condolences on Twitter, adding that he was monitoring the situation with law enforcement. "We send our deepest condolences to the families and friends of those tragically lost, and we pray for the speedy recovery of the wounded."

Vice President Pence addressed the shooting at the beginning of his remarks at a NASA event on Thursday afternoon.

"It's another heartbreaking day for students and parents in America," he said. "In this nation, we mourn with those who mourn, and we grieve with those who grieve. To the families, and to those critically injured, on behalf of the American people, I say from my heart: the hearts of every American are with you today, our prayers are with you, and our prayers are with all the doctors, nurses and hospital staff that are treating your loved ones."

He spoke with President Trump who also conveyed his "deepest sympathies to the families of the victims and the entire Santa Clarita community," Pence said, adding that the president directed the "full resources of the federal government to support local law enforcement."

Pence continued, "Let me say, on behalf of the president, we commend the swift response of local law enforcement and school officials -- they undoubtedly saved lives."

He added, "To every American we say, this president and this administration will remain resolved to bring the scourge of mass shootings to an end and we will not rest or relent until we end this evil in our time and make our schools and our communities safe again."

The William S. Hart Union High School District said all school lockdowns were lifted as of 1:40 p.m. ET. The school district also confirmed that there was only one suspect.

It was not immediately clear if the shooting occurred inside the school building, on school grounds or near the school. It also wasn't clear what time the shooting happened, but a lieutenant with the fire department told Fox News that school began at 6:55 a.m.

"Deputies on scene and still responding," the department had tweeted. "Avoid the area Further information to follow."

President Trump "is monitoring the ongoing reports of a school shooting in Santa Clarita, CA," the White House said in a statement. "The White House encourages all those in the area to follow the advice of local law enforcement and first responders."

Saugus High School is located 40 miles northwest of Los Angeles.

Classes have been canceled for Friday at the William S. Hart Union High School District.

Fox News' Morgan Phillips contributed to this report.