NEW YORK (AP) — Shares of Home Depot sank before the opening bell Tuesday after confirming that its payment systems had been hacked, potentially exposing millions of shoppers who used credit and debit cards at its more than 2,000 U.S. and Canadian stores.

The breach could turn out to be one of the biggest in history. Home Depot did not say how many cards might be affected, but the largest U.S. home improvement chain did say late Monday that its investigation into the breach goes as far back as April.