Cyber criminals who have forced U.S. hospitals, schools and cities to pay hundreds of millions in blackmail or see their computer files destroyed are now targeting the unlikeliest group of victims — local police departments.

Eastern European hackers are hitting law enforcement agencies nationwide with so-called “ransomware” viruses that seize control of a computer system’s files and encrypt them. The hackers then hold the files hostage if the victims don’t pay a ransom online with untraceable digital currency known as Bitcoins. They try to maximize panic with the elements of a real-life hostage crisis, including ransom notes and countdown clocks.

If a ransom is paid, the victim gets an emailed “decryption key” that unlocks the system. If the victim won’t pay, the hackers threaten to delete the files, which they did last year to departments in Alabama and New Hampshire. That means evidence from open cases could be lost or altered, and violent criminals could go free.

Since 2013, hackers have hit departments in at least seven states. Last year, five police and sheriff’s departments in Maine were locked out of their records management systems by hackers demanding ransoms.

Ransomware crimes on all U.S. targets are soaring. In just the first three months of 2016, attacks increased tenfold over the total entire previous year, costing victims more than $200 million. Authorities stress that this number only represents known attacks. One federal law enforcement official told NBC News that the “large majority” of attacks go unreported.

The viruses – most of which come from Russia and Eastern Europe — are typically so impenetrable that even FBI agents have at times advised victims to just pay up and get their data back.

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