KUALA LUMPUR, Malaysia—Prime Minister Najib Razak has told supporters that he isn’t a “crook,” amid continuing fallout from investigations into state investment fund 1Malaysia Development Bhd., or 1MDB.

Pressure has grown on Mr. Najib since The Wall Street Journal reported last year that government investigators had found that hundreds of millions of dollars had entered his personal bank accounts via banks, companies and other entities linked to 1MDB, whose advisory board he chairs.

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