Spokeswoman Molly Snyder declined to comment on further details, such as the vendor's identity or how the hackers stole the credentials, citing the ongoing nature of the investigation.
Snyder did say that since Minneapolis-based Target Corp. confirmed the breach on Dec. 15, it has taken extra precautions such as limiting or updating access to some platforms.
Last week, some card numbers of Target customers from South Texas turned up in the arrest of a pair of Mexican citizens at the U.S.-Mexico border. But experts believe the attack's original perpetrators will be difficult to locate.
Earlier Wednesday, Attorney General Eric Holder said the Justice Department is committed to tracking down the thieves.
In an appearance before the Senate Judiciary Committee, Holder said that the government also will hunt down any people and groups that exploit the stolen data through credit card fraud.
Target says it is working with the Secret Service and the Justice Department in response to the data breach.