Transcript for Terrified students fled, barricaded in classrooms during California school shooting
Reporter: It was just after 7:30 this morning north of Los Angeles - when police cruises and ambulances went screaming through a suburban neighborhood. Authorities say a 16-year-old walked into saugus high school in Santa Clarita carrying a backpack and opened fire. We've got an active shooter, saugus high school. We need all units to respond." Reporter: Today was his birthday, they said. Male Asian wearing a black hat and blue jeans. Reporter: Students were hanging out near the entrance when the alleged gunman approached. From the time he withdrew the handgun, to the time he was on the ground, it was 16 seconds. It's the worst nightmare for a parent to have to go through. Reporter: Off duty officers dropping their children at school became first responders, within seconds on the scene, finding multiple students with gunshot wounds. They tended to the care of all of the victims and got the first aid rolling. So their actions definitely saved lives. You can see another victim being wheeled. Reporter: Those victims at close range, killing a 14 year old boy and 16-year-old girl. I have one done inside the office, and I need paramedics right now. You can see how many fire crews and rescue crews and police officers are behind my. Dozens and dozens, including S.W.A.T. Teams at the school. Parents and others standing by. Some parents received text messages from terrified children U one heartbreaking message saying I don't know what's going on, but I love you and dad so much. The boy's mother responding, I love you, baby. Everything went quiet and then we heard two more. And we realized, something is this is not normal. Then another pause. Then three more, that's when we grabbed our friends and we just ran. I first heard the shots, and then I heard people start running. Everyone was panicked. We turned off the lights and locked the doors. Reporter: A students ran in. One of the students who had one into my class believed she had been shot. Got the gunshot wound which we thankfully have. Jimmy: Yes, classrooms are equipped with kits to treat gunshot wounds. I've never had to treat a gunshot wound as a choir teacher. There's something wrong. I held a bleeding child in the room with 40 sobbing children. I'll never be the same. I yelled at my teacher. Reporter: His parents had been preparing him for this day since he was 4 years old he said. I did have my water bottle just in case. It's like a deadly weapon. Reporter: K-9 units checking abandoned backpacks for explosive devices or weapons. Some students locked in classes for more than two hours before being led to safety. Authorities say the shooter, who is in custody, shot himself and is in critical condition. The weapon that he used was recovered at the scene. It's a .45 caliber semi-automatic pistol which had no more rounds in it. It will be interesting to see where the firearm came from. Did it belong to a parent? Did he borrow it? Was it his? Reporter: There have been at least 30 shootings on school grounds this year. And this morning on capitol hill, senator Chris Murphy, calling on his colleagues to pass universal background checks legislation. We can't go 24 hours without news of another mass shooting somewhere in America. Reporter: And moments later, his fellow Connecticut senator, Richard Blumenthal was urging the passage of the same bill when he was passed a note with the news of the latest tragedy. If we fail to act, as I speak on the floor right now, there is a school shooting in California. How can we turn the other way? How can we refuse to see that shooting in real time demanding our attention? Reporter: And amidest the swarm of rescuers outside that school are terrified parents. My hands are shaking. I was crying. I don't know what to do. Do I go to work? Do I turn around? As parents, you go to work every day to provide for these kids, and the last thing you want to hear is they've taken a bullet. Reporter: The teen's motive is still unclear, but they scouring his social media. It says saugus, have fun at school tomorrow. I can confirm that that was posted on his account, and I can also confirm that it has been changed since this incident. If somebody has altered that, it's really important for law enforcement to figure out who altered it and why. I guess I'll be surprised if they don't find a fair amount of information online. Reporter: And tonight yet another community in America is setting about burying its dead, and again, they are children. We always think these are drills, that they don't really happen here. And unfortunately, it did. And everybody's just in shock. We're all in shock. We're horrified. It's a great community, and for something like this, it's too close to home. Reporter: For "Nightline," Matt Gutman in Santa Clarita, California.
This transcript has been automatically generated and may not be 100% accurate.