ARCADIA  (AP) — The FBI is helping investigate a series of hoax calls that shut down San Gabriel Valley schools three times this week.

Arcadia High School east of Los Angeles was locked down for nearly three hours Thursday afternoon when someone called to report a man with a gun at the school.

A search turned up nothing but Back to School night was cancelled.

An anonymous threatening call by a supposed campus gunman prompted a search Tuesday of Duarte High School and Northview Intermediate School in Duarte.

On Monday, Santa Fe Middle School in Monrovia was searched after a caller claimed he was on campus with an assault rifle.

Authorities say a similar call to West Covina police Monday resulted in brief lockdowns at four hospitals.