A US police department was so determined to get back important files that had been encrypted by the rampaging Cryptolocker Trojan it decided to pay the sizable ransom being demanded by the criminals.

It sounds like a far-fetched and probably serious breach of law enforcement protocol, but according to a local news report, this is exactly what the police department in Swansea Massachusetts decided to do when “several images and word documents,” were found to have been encrypted by the malware.

The department had followed the instructions given by Cryptolocker and on 10 November bought two Bitcoins worth $750 (£470) which resulted in the criminals sending the decrypt key, police said.

“[The Trojan] is so complicated and successful that you have to buy these Bitcoins, which we had never heard of,” said Swansea Police Lt. Gregory Ryan in an admission to the press many will find quite staggering.

Ryan didn’t say why the files were so important that a police department saw fit to pay a digital ransom to criminals, but insisted “It was an education for [those who] had to deal with it,” and that at least the infection had not caused damage to the system the department used for booking official reports and logging photographs.

“We were never compromised,” Ryan said, a statement that many would deem inaccurate.

Only last weekend, the UK National Crime Agency put out an alert that the criminals behind Cryptolocker were now targeting UK SMEs on a large scale. Their recommendation is that affected businesses do not pay the ransom, not least because there is no guarantee that they will even receive an unlock key.

There is growing concern about the scale and success of the Cryptolocker campaign which, it is worth pointing out, is far from the first malware to use the technique of locking or encrypting victim’s files. A key element of Cryptolocker’s recent success is that it has started demanding untraceable Bitcoins for payment rather than more conventional money channels that were easier to block or trace.

Another weakness is that there is often no central place for affected individuals to report infections, nor seek advice. Consequently, some victims pay up. The citizens of Swansea Massachusetts now know that this helplessness includes their local police department.

"With the FBI and the UK NCA stating that this type of activity should not be encouraged by paying the ransom, it is surprising to see that the local police department paying to regain access to the files,” commented Gavin Millard, EMEA technical director of security firm Tripwire.

“What is more concerning though, is the apparent lack of security and backup procedures on systems that could be storing critical and highly confidential documents.”