WILMER (CBSDFW.COM) – A small North Texas city is among nearly two dozen across the state that have been paralyzed by a major cyber attack.

In Wilmer, municipal computers are being held for ransom — most likely from hackers in Russia or China.

A uniformed National Guard IT expert has been working non-stop on Wilmer’s computers since the attack, but the police department, water department, and even library computers in the city have been corrupted.

While the town’s entire network is shutdown, leaders in Wilmer are assuring the public that personal information has only been frozen, not stolen.

The hackers demanded a ransom paid in bitcoin currency in exchange for release of the compromised data that’s been seized.

This particular cyber attack is happening in Wilmer and more than 20 other Texas cities and towns where workers woke up Friday to blue computer screens with the message “all your files are encrypted”.

CBS 11 News has learned that some of the other affected Texas municipalities include nearby Kaufman, the panhandle city of Borger, and the Johnson County city of Keene, which is about 30 miles south of Fort Worth.

Cybersecurity expert Keith Barthold says it’s no coincidence that the hackers targeted particular cities. “It makes sense that it’s the smaller towns that are being attacked, because typically their cyber defenses are going to be a lot leaner, budgets are lower,  they’re not aware, and they just don’t have the staff to put a framework in place to defend against these kind of attacks.”

Barthold thinks the hackers probably gained access to the computer system through a phishing email or a compromised password.

Wilmer has a population of just 3,600. Mayor Dr. Emmanuel Wealthy Williams told CBS 11 that an assessment was expected to be completed Tuesday, but that it would probably be weeks before city operations are back to normal.