A British man who hacked TalkTalk Telecom Group Plc, blackmailed executives and offered to supply customer data has been jailed for four years.

Daniel Kelley, 22, was sentenced after pleading guilty to 11 charges of hacking and blackmailing TalkTalk, London police said in a statement Monday. The cyber attack of the British telecoms firm took place in 2015, leaving around 20,000 accounts compromised.

Kelley demanded TalkTalk Chief Executive Officer Dido Harding and other TalkTalk employees pay him 465 bitcoin, or about $362,000, as part of his hacking attempt in October 2015, prosecutors said in 2016. Kelley also hacked into half a dozen other companies and organizations including a Welsh college and an Australian education authority.

“Kelley is a prolific and ruthless cyber criminal and blackmailer who caused considerable damage, distress, harm and loss to victims’ worldwide,” Rob Burrows, an officer in the Metropolitan Police’s Cyber Crime Unit, said in a statement.

TalkTalk told its customers in October 2015 that about 20,000 bank account numbers had been compromised in a cyber attack, and has estimated that the breach would cost 35 million pounds ($44 million).

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