The U.S. Department of Justice is investigating the Target credit and debit card information security breach, Target said on Monday, the Associated Press reported.

The inspection comes after Target disclosed last week that the credit and debit card information of 40 million of its customers were stolen for a period of three weeks between Nov. 27 and Dec. 15. Target said they were able to resolve the issue.

The Justice Department did not confirm their investigation, but Target said the Department has its full cooperation, the AP reported.

Target is still unsure exactly how their security was infiltrated. Target, the second largest discount chain in the U.S., said it is partnering with the Secret Service in its own inquiry. Target's general counsel and state attorneys general discussed the breach in a conference call on Monday, the AP reported.

"Target remains committed to sharing information about the recent data breach with all who are impacted," Molly Snyder, a target spokeswoman, told the AP.

Almost all of Target's 1,797 stores across the U.S. were affected. Hackers were somehow able to insert software at point-of-sales stations where the magnetic strips on credit and debit cards are swiped.

Target issued an apology to its customers and offered free credit card protection services and a 10 percent discount to shoppers over the weekend.

But by Monday several customers already filed lawsuits against Target, the AP reported, accusing Target of negligence.

One Target customer, Scotty Haywood, told the AP he realized his information had been compromised when his wife tried to use his debit card at a grocery store. Haywood, who lives in Alabama, said there was only $2,200 in the bank account when they later checked it.

Haywood said he had used his debit card at a Target the day after Thanksgiving. He told the AP he will no longer shop there.

"The possible savings of a few dollars (by going to Target) are nothing compared to the money that has been stolen form us," Haywood told the AP.