EDISON - Over 20 bomb threats called in to schools in Bergen and Passaic counties forced the evacuations of thousands of students Wednesday.

The threats were called to the schools in several towns, including Midland Park, Hawthorne, Fair Lawn, Elmwood Park, Little Falls, Lodi, Englewood, Leonia, Hackensack and Fort Lee.  Some of the districts received threats for more than one school.

The schools reported that a "robotic" sounding voice called the schools from an anonymous number.

Law enforcement agencies came to the schools with bomb sniffing dogs to investigate the claims. No devices or threats were found after police swept the schools.

The students were allowed to resume classes later in the day.

This is the latest incident in recent months where multiple towns have received fake bomb threats.

Wednesday’s threats came just one day after the Passaic County Bomb Squad gave a presentation to Passaic High School students about the consequences of creating hoax calls.

Passaic County Sheriff’s Detective George Rosenthal says that each hoax bomb threat costs taxpayers about $10,000.

Bergen County Prosecutor’s Office spokeswoman Maureen Parenta said in a statement, “[Bomb threats and hoaxes] as you know, have been an ongoing investigation for us. We are going to see how today’s events fit in to prior events in our area.”

Federal authorities are involved in the investigation for the previous incidents.