There were multiple domestic official investigations into the 1MDB fund, including a special task force headed by the attorney general. The team raided the office of 1MDB and took away some documents.
But in July 2015 Mr Najib replaced his deputy Muhyiddin Yassin, who had criticised Mr Najib's handling of allegations, and removed the attorney general on health grounds. The investigation was temporarily suspended.
However, foreign authorities also began investigating.
Singapore police said they had frozen two bank accounts as part of an investigation linked to 1MDB, and Hong Kong police said they were investigating bank deposits allegedly linked to Mr Najib.
Swiss authorities opened criminal proceedings related to 1MDB on "suspected corruption of public foreign officials, dishonest management of public interests and money laundering".
The Wall Street Journal said the Federal Bureau of Investigation had also launched an investigation into allegations of money-laundering at 1MDB. The New York Times said a US federal grand jury was examining the claims involving Mr Najib.
However, the president of 1MDB Arul Kanda told the BBC that no foreign investigators had contacted the investment fund and no employees were under investigation.