Mira Costa High School was closed for a second day Wednesday after threats against the school were posted on the anonymous messaging app Yik Yak. Police say they don't yet know who sent the messages.
The messages were posted Monday, and Manhattan Beach Unified School District Supt. Michael Matthews decided to close the school Tuesday to investigate the threats.
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 in the 1400 block of Artesia Boulevard was locked down Monday after a student notified a teacher about a "vague and non-specific" 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 in the case, 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.
This week, an Illinois high school also beefed up security in response to a Yik Yak threat.
In March, authorities investigated a bomb threat made on Yik Yak that led to a lockdown at San Clemente High School. They have not discovered who was responsible for that message.
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 have also blocked access to Yik Yak from the school's Wi-Fi network.