A breach would be the latest high profile attack on national retailers, following hits on Target (TGT), Neiman Marcus and Sally Beauty, among others, in the last year. For Target, the breach hit last year at the peak of the holiday shopping season and hurt sales for months and has put all retailers on edge.

“Our internal teams and third parties have been working around the clock to find the breach,” Blake said at the Goldman Sachs Global Retail Conference in New York.

Home Depot found out about the potential breach on Tuesday, Blake said. It was first reported on the KrebsOnSecurity.com blog on Tuesday, which reported that multiple banks were seeing evidence that Home Depot stores may have been the source of a massive new batch of stolen credit and debit cards that went on sale that morning on the black market. Home Depot said on Wednesday it was working with law enforcement agencies to investigate the breach and had hired security firms Symantec Corp. and Fishnet Security to help.

Blake called cyber security “a major issue” for Home Depot and said that the retailer has invested in new credit-card terminals at cash registers that take cards with computer chips that create unique codes for each transaction and are thus considered safer for consumers than the magnetic strips on the back of current cards. Home Depot will activate the chip-reading technology on the terminals by the end of 2014, well ahead of a retail industry-wide deadline of October 2015, Blake said. He reiterated that customers would not be on the hook for fraudulent charges and would get free credit monitoring if a breach did indeed occur, but urged them to keep an eye out for unusual activity in their accounts.

“The most important thing for us is making sure that our customers feel comfortable shopping at The Home Depot, and that’s going to be our guiding principle,” said Blake, who announced his upcoming resignation before the possible breach came to light.

Just look at Target’s travails to see why he feels that way.