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A bomb threat was called into Alhambra High School around 11:40 a.m., police report.
Hundreds of students have poured onto the track field as the school is placed on lockdown pending an investigation. Alhambra police officers are going door to door with a canine unit, checking rooms throughout the campus for the reported "suspicious package."
Sgt. Jerry Johnson, a spokesman with the Alhambra Police Department, has confirmed that there was a call about a possible mysterious package on campus. KTLA reports that the call was placed by a "young person" and possible student, but Johnson could not confirm.
No devices have been found as of 2:00 p.m.
It's unknown how long the lockdown will last.
This story has been updated.