Singapore's economic crime unit and city prosecutors have interviewed current and former Goldman Sachs executives who worked on bond offerings from 1Malaysia Development Bhd, Bloomberg reported.
Najib has denied any wrongdoing.
A Goldman Sachs spokesman in Hong Kong declined to comment. Officials at the Singaporean police and 1MDB were not immediately available for comment.
The Monetary Authority of Singapore had earlier in the week barred two individuals involved in breaches related to Malaysia's 1MDB fund from taking part in financial services management and advisory activities.
In March, the MAS issued a 10-year prohibition order against Tim Leissner, former Southeast Asia chairman at Goldman Sachs, who it said had made false statements on behalf of his bank without its knowledge or consent.
Leissner was responsible for managing the relationship with Malaysia's 1MDB fund when Goldman Sachs was engaged to arrange three bond issues from 2012 to 2013. MAS had flagged its intention to ban Leissner last December.