The FBI is now joining in the investigation into a string of threats to multiple San Gabriel Valley schools and at least one hospital this week.
For several hours Thursday, police scoured the campus of Arcadia High School, looking for a possible gunman.
Lt. Bob Anderson says, "The Arcadia PD received an anonymous call of someone on campus with an assault rifle."
The school was locked down for hours until the all-clear was given.
Just yesterday, police responded to another threat called into Duarte High School and Northview Intermediate School which shares the same campus.
The Sheriff's department says someone claiming to be a student at the middle school was on campus with a gun.
But deputies lifted the lock down after a search of the campus turned up empty.
On Monday, police responded to Santa Fe Middle School with guns drawn after two threatening calls came in targeting the school. Police say in one, the caller threatened to shoot the kids on campus and in the second, the caller threatened to shoot if police came to the school. But after police searched the campus, kids were cleared to go home.
Monrovia police chief, Jim Hunt said, "there's no sign of any gunman, we're doing everything we can to make sure the kids are safe."
The FBI is now aiding the investigation into these threats because it spans multiple cities but would not comment any further. Law enforcement agencies say the same person may be responsible for all the threats Says Anderson.