CHARLOTTE, N.C. (AP) _ Federal officials are investigating whether drug money laundering is behind the phenomenal growth in the cash surplus at Charlotte's Federal Reserve Bank.

The branch saw its surplus cash skyrocket from $53 million in 1990 to $968 million in 1991.

''A lot of it is legitimate money,'' said Wolfgang Preisler of the Drug Enforcement Administration. ''But there certainly is some drug money. We've seen a lot of high-cost purchases, like shopping centers and golf courses.''

DEA agents have visited North Carolina mountain resort towns to warn local real estate businesses about possible money laundering activities.

Money laundering occurs when illegal money is run through the bank accounts of legitimate businesses to make it look clean.

The surplus at the Fed branch in Charlotte, North Carolina's largest city, ranks fifth in the nation. By comparison, its 1988 surplus was $28 million.