After the threat was reported the school went into immediate lockdown, following standard protocol, as did nearby Monroe Elementary.
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:
"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."