South Pasadena High School was placed on lockdown Friday after district officials received a "communication about potential suspicious activity" involving the campus.
School officials did not provide details about the communication but said in an email to parents that they "do not believe there is an imminent issue."
Principal Janet Anderson and South Pasadena Unified School District Supt. Geoff Yantz said they were "erring on the side of caution" and were working with local police.
The communication, they said, was not related to the alleged threat made by two South Pasadena High students charged with planning a mass shooting on campus in September.
"There in no relationship between this issue and the incident that started our school year," school officials said.
At the time, police said the teenagers developed a plan to carry out a mass shooting and kill "as many people as possible."
The students allegedly swapped ideas over Skype about using explosives and firearms to target certain school staff members and kill classmates.
In the email sent to parents Friday, district officials said they planned to provide additional details in a subsequent email or notify parents whether students would be released from school.