(Bloomberg) -- Prolific criminal hackers were behind a cyberattack on the UK’s Royal Mail that has shut down its ability to send international letters and parcels, according to two people familiar with the matter.

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A ransomware gang known as LockBit targeted the British business and used encryption to lock some of its computers, rendering them inoperable, according to the people who asked not to be identified because the matter isn’t public. The gang usually demands payment to unlock computers it has compromised and often threatens to leak stolen data to pressure victims to pay. It’s not known how much money the group has demanded from Royal Mail or whether the company intends to pay.

Royal Mail declined to comment. A representative for LockBit didn’t respond to a message seeking comment.

The company, part of International Distributions Services Plc, said in a statement on Wednesday that it was experiencing a “cyber incident” that was causing severe disruption to international export services.

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“We are temporarily unable to dispatch items to overseas destinations. We strongly recommend that you temporarily hold any export mail items while we work to resolve the issue,” the company said in the statement. “We immediately launched an investigation into the incident and we are working with external experts. We have reported the incident to our regulators and the relevant security authorities.”

The UK’s National Cyber Security Centre said it was aware of an incident affecting Royal Mail Group Ltd. and was working with the company, alongside the National Crime Agency, to fully understand the impact.