The FBI has concluded a two-year investigation into an online market for stolen credit card data behind millions of dollars worth of fraud.

The Dark Market forum was buying and selling stolen financial information, including credit card data, log-in credentials (user names and passwords), as well as equipment used in carrying out certain financial crimes.

The FBI operation, carried out in conjunction with the UK's Serious Organised Crime Agency and other law enforcement agencies around the globe, has resulted in 56 arrests.

However, the FBI believes that there may have been as many as 2,500 regular forum members.

"In today's world of rapidly expanding technology where cyber-crimes are perpetrated instantly from anywhere in the world, law enforcement needs to be flexible and creative in its efforts to target these criminals," said FBI cyber division assistant director Shawn Henry.

"By joining forces with our international law enforcement counterparts we have been, and will continue to be, successful in arresting those individuals and dismantling these forums."

The FBI estimates that it has protected bank accounts and credit lines worth $70m by shutting down the forum, and said that the investigation has spawned new leads and investigations that will also be followed up.