New Delhi - It is the latest in a string of leaks over the years that have exposed secret deals, money laundering and financial crime.
These documents are some of the international banking system's most closely guarded secrets. Banks use them to report suspicious behaviour but they are not proof of wrongdoing or crime.
FinCEN is the US Financial Crimes Investigation Network. Concerns about transactions made in US dollars need to be sent to FinCEN, even if they took place outside the US.
Suspicious activity reports, or SARs, are an example of how those concerns are recorded. A bank must fill in one of these reports if it is worried one of its clients might be up to no good. The report is sent to the authorities, BBC said.
It has been revealed through these documents that a bank allowed fraudsters to move millions of dollars even after it learned from US investigators that the scheme was a scam.
JP Morgan allowed a company to move more than $1 billion through a London account without knowing who owned it. The bank later discovered the company might be owned by a mobster on the FBI's 10 Most Wanted list.
Accoridng to BBC, the UK is called a "higher risk jurisdiction" like Cyprus, according to the intelligence Division of FinCEN. That's because of the number of UK registered companies that appear in the SARs. Over 3,000 UK companies are named in the FinCEN files - more than any other country.
There have been a number of big leaks of financial information in recent years, including 2017 Paradise Papers. The 2016 Panama Papers - Leaked documents from the law firm Mossack Fonseca showed more about how wealthy people are using offshore tax regimes, the BBC said.