Investigators now think that overseas hackers are to blame for a major data breach that compromised 40 million debit and credit cards used at Target stores the first three weeks of the holiday season.

An unidentified source said that the government investigators don't believe the hackers had inside assistance from a Target worker, according to Reuters.

The source has not confirmed how the overseas hackers were able to get the stolen information, and investigators have also not confirmed where the hackers are based out of. This could be because investigators don't want the crooks to know they've figured out who committed the crime to allow them to destroy potential evidence.

Data stolen from Target has been "flooding underground markets" that sell stolen credit cards, said Brian Krebs, who first broke the story, according to Reuters.

The breach started right around when Black Friday deals started on Nov. 27 and ended around Dec. 15 according to Steinhafel.

KrebsOnSecurity.com reported on Friday that cards stolen from Target were being offered at "card shops" for rates starting at $20 each and going to more than $100.

Target confirmed the breach on Dec. 19, a day after Krebs broke the story. It has declined to say how the breach occurred in the first place however.

Yaron Samid, chief executive officer of BillGuard has warned customers not to look for big charges but for smaller ones, ranging from $1 to $1,000. 

"This could be something that hits your card months from now, so you need to continue to be vigilant," says Yaron Samid, chief executive officer of BillGuard, a company that offers a free service monitoring credit and debit cards for unusual activity.

Target is based in Minneapolis and has over 1,700 stores in the U.S. and 24 in Canada.

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