Authorities said Wednesday that they have identified a female juvenile who is responsible for at least one of the threats against Mira Costa High School that were posted on the anonymous messaging app Yik Yak.
The girl, whose name was not released because of her age, is not a student in the Manhattan Beach Unified School District, authorities said.
She was interviewed by investigators but is not in custody, according to the Manhattan Beach Police Department.
Police said they will continue to investigate the threats to determine whether others were involved.
The high school was closed for a second day Wednesday after the threats prompted district Supt. Michael Matthews to close the school Tuesday.
Classes are scheduled to resume Thursday and campus security will be increased, district administrators said.
"We look forward to welcoming all students back to campus, but understand that parents/guardians have the right to make an informed decision for their child," the district said in a statement.
Authorities said the high school was locked down Monday after a student notified a teacher about a "vague and nonspecific" message posted on Yik Yak, a popular app on Apple and Android devices.
The message read: "If you go to Costa, you should watch out very closely at school today."
Police and school officials searched the campus but did not find any additional evidence, according to the district.
Later that afternoon, a second message said "nice try Costa, today was just a drill," said Manhattan Beach Police Officer Stephanie Martin.
Officials said a third message appeared about 8 p.m.: "tomorrow at 2 be ready Costa, you're going on lockdown."
Yik Yak has come under scrutiny after anonymous messages prompted lockdowns and closures at schools in California and other parts of the country.
Manhattan Beach school administrators said they have asked Yik Yak for geo-fencing security software, which would "create a boundary" around the district's campuses to make the app unusable there.
District officials also have blocked access to Yik Yak from the school's Wi-Fi network.