The Los Angeles County Sheriff said the suspect had been detained near Highland High School in Palmdale, 40 miles (65 kilometers) north of downtown Los Angeles.
Dr. Petrik said the reports of multiple casualties and dozens of injuries caused the hospital to virtually shut down the operating and emergency rooms for an onslaught of victims.
The suspect called his father after the shooting, tell his dad he fired the gun in the air.
Frightened parents and students embraced outside the gates of Highland High late Friday morning.
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About a half hour after the Highland calls, gunshots were reported at the Manzanita Elementary School, located about seven miles away from Highland High School.
Reports of the shooting and an ultimately false report of another attack at a nearby elementary school sparked chaos, terrifying parents and drawing a massive police response in the high desert city of Palmdale in Southern California.
Firefighter-paramedics told NBC Los Angeles they received a report of one person with a gunshot wound, though any injuries couldn't be independently confirmed. Both were students at the school. He didn't elaborate. The injured student is expected to make a full recovery.
George Ramirez, 15, said he saw students running on campus earlier that morning, but was confused as to why.
Betsy Sanchez, a spokeswoman for the Antelope Valley Union High School District, said the office is in contact with victim's family.
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The parents recently transferred the boy from Highland High School though it was unclear why. He said he heard one shot and turned.
"I wish I could tell you what his motivation was or what he was looking to do", McDonnell said.
Friday's incident comes during a time when gun control and school safety has been a national hot topic since the Parkland shooting on February 14.
"We barricaded the room", she said.
Sergeant Bob Boese told the OC Register that this appears to be an isolated incident and that the suspect was a former student at the school who ran away after shooting one student.
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