An American man who stole millions of credit card numbers in one of the biggest computer hacking operations in US history was sentenced Thursday to 20 years prison. Albert Gonzalez, 28, pleaded guilty last September to separate cases related to hacking into the computers of big retailers including TJX Cos and BJ's Wholesale Club. Sentencing him in Boston, Judge Patti Saris said: 'This is the largest and most costly example of computer hacking in US history.' The sentence was less than the 25 years demanded by prosecutors. Saris said several factors were taken into consideration. 'I have read many letters from people who love you. However I have also read of your macho glee on how you can beat the system,' the judge said. 'I need to send a message balancing everything, considering the huge corporate cost at the high end, and also if you do seem remorseful and you did give up the one million (dollars) buried in your parents backyard.' Gonzalez, a computer expert, told the court: 'I stand before you humbled by these past 22 months of incarceration. I am guilty.' He faces sentencing in other cases Friday. More than 250 financial institutions were targeted in the scam, also including Heartland Payment Systems, a New Jersey-based card payment processor; 7-Eleven Inc., a Texas-based nationwide convenience store chain; and Hannaford Brothers Co. Inc., a Maine-based supermarket chain. Gonzalez was accused of leasing servers to other cyber-thieves who used the platforms to store malicious software known as 'malware' and launch attacks against corporate victims. 