KUALA LUMPUR, Malaysia—Malaysia’s attorney general said an official investigation into a troubled state investment fund has uncovered documents related to allegations that money was transferred into the personal bank accounts of Prime Minister Najib Razak.

A task force comprising the central bank, the national police and the nation’s anticorruption agency uncovered the documents during a probe of 1Malaysia Development Bhd., or 1MDB, Abdul Gani Patail, the attorney general, said Saturday.