After the threat was reported the school went into immediate lockdown, following standard protocol, as did nearby Monroe Elementary.
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Police searched the area and found no evidence of an active shooter, declaring the situation clear.
Authorities believe it may have been a "swatting" incident where a fake call is made to law enforcement -- noting that other agencies in the area had received similar threatening calls.
Monrovia Unified School District Superintendent Ryan Smith informed the school community there was no evidence of a credible threat, writing:
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"It appears as though this is not a credible threat. There is no evidence of an active shooter at the high school and there has been no violence of any kind. This may have been a "swatting" incident where a fake call is made to law enforcement in the hopes of eliciting a massive emergency response. We have heard that schools in other districts may have received similar calls."