Questions about movement of funds from 1MDB came to widespread attention nearly a year ago, when the Wall Street Journal reported that in 2013 nearly $700 million had flowed from the debt-ridden fund to Najib's personal bank account.
Najib has repeatedly denied wrongdoing and, under pressure from the outcry caused by the WSJ report, said the funds were a private donation from a Middle Eastern country he declined to name. He has denied benefiting personally from the funds.
In January, Malaysia's Attorney General Mohamed Apandi Ali said that Saudi Arabia's royal family gave Najib a $681 million gift, of which Apandi said about $600 million was later returned.
Apandi said that no criminal offense had been committed. But globally, investigations into 1MDB in locales as varied as US, Switzerland, Singapore and the Seychelles have continued.
Last month, the MAS ordered BSI to shut down its Singapore operations, with prosecutors in the city-state and in Switzerland weighing criminal charges against the bank.
While Singapore's authorities did not mention 1MDB at the time of BSI's closure, Swiss authorities said the move was related to its investigation of the fund.
The MAS said the Switzerland-based bank had lost its status as a merchant bank in Singapore due to "serious breaches of anti-money laundering requirements, poor management oversight of the bank's operations, and gross misconduct by some of the bank's staff." It was the first time since 1984 that MAS had withdrawn a merchant bank's approval.
In a related development, Abu Dhabi wealth fund International Petroleum Investment Company (IPIC) said in a London Stock Exchange filing on Tuesday that it would seek arbitration in London courts over what it called 1MDB's failure to make contractual payments. The Abu Dhabi fund claimed it was seeking around $6.5 billion from 1MDB.
Last year, a 1MDB unit transferred funds owed to IPIC to a British Virgin Islands entity called "Aabar Investments PJS Ltd.," which has a name closely resembling Aabar Investments PJS, an IPIC subsidiary, according to a Wall Street Journal report.
The Abu Dhabi fund said it didn't receive the payments.