TEWKSBURY (CBS) — Tewksbury police say they were the victims of a hacking, forcing officers to negotiate with “cyber terrorists.”

Eventually, they had to pay a ransom to get the department’s files unencrypted.

Police Chief Timothy Sheehan said this was a “gradual” attack that began on a weekend.

“It had encrypted much of the data and made it inaccessible as far as for our dispatchers and for our police officers and for the firefighters and fire stations,” he said. “By Monday morning, we really became aware that we had some issues.”

Even computer specialists from the FBI couldn’t break the cyber hold the hackers had, so Tewksbury police paid a $500 ransom using Bitcoin.

“We certainly don’t advocate negotiating with cyber terrorists but this happened to be the perfect storm,” Sheehan said.

Bitcoin is the digital currency popular with crooks because it’s nearly impossible to trace.

“We wanted to do the absolute best that we possibly could,” Sheehan said. “Unfortunately no one had any luck with tying this back to any one individual.”

No information was stolen or compromised, he said.