Officers responded to a report of seven shots being fired at the campus on Marengo Avenue, according to the Pasadena Police Department.
That triggered a campus lockdown, but police did not find any evidence of a shooting. The department said it appeared to be a hoax.
Dr. Amy McGinnis, the school's principal, issued the following message:
"In an abundance of caution, Blair High School has been placed on lockdown in response to what appears to be a swatting incident. Pasadena Police are on site and conducting a thorough search of the campus. We anticipate receiving an all clear very soon. Students and staff are safe. Swatting is an intentional report of a false emergency."
McGinnis later said that all students and staff were safe, and police cleared the campus.