The sheriff said the brother is facing charges of possession of an assault weapon and other violations and that the teen had an extensive disciplinary history at school.
School threats have been increasing in the area since a shooting last week at a Florida high school killed 17 people, McDonnell said, adding that "this should be a wake-up call for all of us."
School district security officer Marino Chavez told reporters that when he heard the threat, he asked the student about it, and the teen confirmed that he made it but was just kidding and didn't mean it. Chavez told the student he could not say such things at school.
Robert Jacobsen, general counsel for the Norwalk-La Mirada Unified School District, said the teen didn't like a teacher's rule banning headphones in class but declined to provide further details about him, citing privacy issues.
"They felt there was enough there that they should call law enforcement so they can investigate further," Jacobsen said. "In this day and age, we have to be proactive and make that report and go from there."
He said the safety of students and staff is the highest priority.
"Given shootings that happened in Florida, and we hear about quite a few of them, we're all looking to make sure we can prevent these concerns and find out what's going on," he said.
Superintendent Hasmik Danielian said in a statement that "we responded quickly and effectively when we first learned about the potentially dangerous threat that was made by the student."
"We will remain vigilant in our efforts to make sure that we are doing everything possible when it comes to safety and security for our entire school community," she said.
Associated Press writer Amanda Lee Myers contributed to this report.