Valve has informed users of its Steam online game distribution platform that hackers have probably downloaded encrypted credit card transaction data from a backup database during an intrusion last year.

In November 2011, Valve announced that hackers gained unauthorised access to Steam's user database, but said that there was no evidence to suggest a leak of encrypted credit card details at that time.

However, that has since changed. "Recently we learned that it is probable that the intruders obtained a copy of a backup file with information about Steam transactions between 2004 and 2008," said Gabe Newell, Valve's co-founder and managing director, in an email sent to Steam users.

According to Newell, the backup file contained Steam user names, email addresses, encrypted credit card details and encrypted billing addresses, but no account passwords.

Valve doesn't have reasons to believe that the sensitive transaction data was decrypted, Newell said. However, this possibility should not be excluded.

"As I said in November it's a good idea to watch your credit card activity and statements," Newell said. "And of course keeping Steam Guard [a Steam account security system] on is a good idea as well."

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The company decided to send the email to Steam users before sending official breach notification letters as required by some states when credit card information is involved.

The investigation into the intrusion is ongoing, but authorities might already have a suspect in custody.

During a recent conference call between the FBI and the Metropolitan Police, law enforcement officials discussed the case of a 15-year-old hacker known online as TehWongZ, who was arrested before Christmas.

Metropolitan Police officials said that TehWongZ took credit for breaking into Steam in a written statement that listed his hacking achievements. The hacker allegedly said that he downloaded 32,000 Steam usernames, passwords and credit card details.