(CNBC) -   The FBI is probing a bankrupt California wine seller that some customers claim was operating a multimillion-dollar Ponzi scheme. 

In a statement, the law enforcement agency said Wednesday that is seeking information from customers of Berkeley-based Premier Cru, which filed for bankruptcy last month.

"The FBI is investigating claims of a Ponzi scheme involving the Berkeley wine company Premier Cru," FBI spokeswoman Michele Ernst told CNBC in a statement. "Due to the wide scope and high number of complaints from people who claim to have been impacted by Premier Cru's bankruptcy, the FBI has established an email address for individuals to notify us with complaints, concerns and tips. It is [email protected] "

The principal owner of Premier Cru, John Fox, couldn't be reached for comment, nor could his attorney. Fox also has filed for personal bankruptcy.

The fall of Premier Cru is believed to be the biggest wine seller bankruptcy in recent history, and has shaken confidence among wine buyers. The company filed for Chapter 7 liquidation in January, after several customers filed lawsuits claiming it was operating a Ponzi scheme by taking their money, but never sending their wine.

Premier Cru's bankruptcy statement listed $7 million in assets and $70 million in debts, much of it owed to more than 9,000 customers. Those customers include billionaires such as William Koch and Jeff Greene as well as several Chinese collectors, according to court documents.

But Mark Bostick, trustee for the bankruptcy, said the largest creditor may turn out to be American Express. Several customers who purchased wine at Premier Cru used their AmEx card to do so, and the company has been issuing them refunds.

American Express declined to comment on the case, but said that generally, card members who use their cards "to purchase goods or services that have not yet been delivered from a merchant that files for bankruptcy protection can call the number on the back of their card to determine if they are eligible for a refund."

Wealthy wine collectors around the world bought wine, wine futures and pre-arrival wine from Premier Cru. In the wine futures market, customers buy wine that is aging but has not yet been bottled, in hopes of getting it cheaper than the post-bottle price. Pre-arrival wine is that which is bottled but has not yet hit store shelves.