By Bill Shields

BOSTON (CBS) – The Haverhill School Department is just the latest local victim of cyber hackers, and ransomware. Cyber criminals get into a computer network, and freeze it, then demand money to free it up.

“It basically encrypts your data on a computer and makes it unusable, until you pay the ransom,” said FBI Special Agent Dave Farrell.

READ MORE: 'Had A Heart Of Gold': Construction Worker Killed In Newton Identified As Russell Harron

The cyber-criminals have targeted municipal computer systems, schools, even police departments, demanding money to get their systems working again.

“What is it that you have that is most valuable to you, is what these ransomware actors are interested in,” Farrell said. “They’re interested in a quick payday.”

READ MORE: Mother's Day Brings Hope To Businesses Impacted By Pandemic

In Plymouth, they have spent time and money on training to make sure the ransomware doesn’t get them.

“That helps put all of us, the staff, every one of us, in a better position to spot threats, it changes our behavior so we’re looking for that, we’re not as trusting,” said Plymouth Town Manager Melissa Arrighi.

The FBI said one of the keys to defeating the malware is to have duplicates of your data.

MORE NEWS: Worcester Pizza Shop Struggling To Find Staff As Customers Fill Dining Room

“We’re always preaching the companies that have not had to pay the ransom in the past had backups,” Farrell said. “They back up their data.”

Bill Shields