Home Depot Inc. fell as much as 3.4 percent in New York trading after saying it was working with banks and law enforcement to investigate a possible data breach.

“We’re looking into some unusual activity,” Paula Drake, a spokeswoman for the Atlanta-based company, said in an e-mailed statement. “We are aggressively gathering facts at this point while working to protect customers. If we confirm that a breach has occurred, we will make sure customers are notified immediately.”

Brian Krebs, the independent journalist who uncovered a hacker attack at Target Corp. last year, reported that a “massive” batch of stolen credit- and debit-card information went on sale this morning. There’s evidence that the cards are linked to Home Depot stores, Krebs said on his website, KrebsOnSecurity.

Home Depot shares dropped as low as $89.85 in New York. The stock had climbed 14 percent this year before today.