Coinbase, the largest cryptocurrency exchange based in the U.S., said Thursday that criminals had improperly obtained personal data on the exchange’s customers for use in crypto-stealing scams and were demanding a $20 million payment not to publicly release the info.

Coinbase says it will seek the harshest penalties against the crooks, who stole tens of thousands of client records.

Coinbase didn't say how many customers had their data stolen, but warned clients they should be on alert for scams targeting them for additional details.