By Mark Hosenball and Dhanya Skariachan WASHINGTON/NEW YORK (Reuters) - The U.S. Secret Service visited the office of a Pennsylvania-based refrigeration contractor, Fazio Mechanical Services, in connection with an investigation of the Target Corp consumer data breach, a spokesman for the agency said on Wednesday. Fazio is the largest refrigeration contractor in the Western Pennsylvania region, and Target is one of its clients, according to the Sharpsburg-based company's website. A law enforcement source told Reuters that evidence suggests the hackers stole login credentials from Fazio and may have used the credentials to break into Target's network. The source added, however, that investigators were not sure that this was what happened, and it was possible the hackers used other ways to breach Target's network. Representatives for Fazio could not be reached for comment. Security blogger Brian Krebs reported earlier on Wednesday that Fazio President Ross Fazio had confirmed the visit by the Secret Service in connection with the Target probe. Last week, Target, the third-largest U.S. retailer, said the theft of a vendor's credentials helped cyber criminals pull off a massive data breach during the holiday shopping season in late 2013. Target did not identify the vendor. "Because this continues to be an active and ongoing investigation we don't have additional details to share at this time," Target spokeswoman Molly Snyder said in an email on Wednesday. Target previously said that hackers stole about 40 million credit and debit card records, as well as personal information - such as addresses and phone numbers - of about 70 million customers. The attack, followed by disclosures of data breaches at Neiman Marcus and other retailers, has renewed concerns about the security of payment card networks. (Reporting by Mark Hosenball in Washington D.C. and Dhanya Skariachan in New York; Editing by Dan Grebler)