A network of secret slush funds inside a corrupt Cyprus bank has exposed Kremlin connections to frontmen for Syria’s chemical weapons programme and the terrorist group ISIS.

The web of accounts at FBME Bank, revealed in an explosive cache of leaked documents, also moved hundreds of millions of dollars from suspect Moscow-based figures including associates of the Russian president Vladimir Putin, mafia figures, and Kremlin officials.

In recent years, a series of offshore banking leaks have revealed how the super-rich can avoid paying taxes by hiding their wealth behind complex structures that – while morally dubious – are largely legal. But the FBME files paint a darker picture, revealing evidence that some of the world’s worst criminals have been allowed to move their money.

FBME, the focus of an ongoing BuzzFeed News investigation, was shut out of the US financial system this year after the Financial Crimes Enforcement Network, known as FinCEN, declared it a “primary money laundering concern” that had attracted “illicit finance from the darkest corners of the criminal underworld”. FinCEN kept much of its evidence against FBME secret – but the files obtained by BuzzFeed News show a host of alarming transactions that ran through the bank for years and that point to one place: Russia.

Yesterday BuzzFeed News revealed how heavyweight international law firms Quinn Emanuel Urquhart & Sullivan and Hogan Lovells – including a Quinn Emanuel partner defending two key Trump officials in the Russia inquiry – did battle for FBME, while accountants at two of the world’s biggest accountancy firms, KPMG and Ernst & Young, gave the bank their gold-plated seals of approval despite glaring red flags. Tomorrow, we will reveal how a major Western bank helped FBME funnel hundreds of millions of dollars of suspicious money into the US.

Both law firms vigorously defended their work for FBME, which they said conformed to the highest standards of legal ethics. William Burck, Quinn Emanuel’s co-chair of white-collar crime, said he had little knowledge of FBME’s Russian business and that his work for the bank had no connection to his new role representing White House counsel Donald McGahn and former chief of staff Reince Priebus in the Kremlin collusion inquiry. The two accountancy firms refused to comment, citing client confidentiality.