KUALA LUMPUR, Malaysia—Officials here are investigating whether the government of former Prime Minister Najib Razak used funds from a China-backed infrastructure program to help pay debts owed by the state investment fund at the center of one of the world’s biggest graft probes.

Prime Minister Mahathir Mohamad in July froze over $20 billion in Malaysian infrastructure projects that are part of China’s One Belt One Road initiative, saying the contract values appeared to be inflated, in what he called a sign of possible corruption.

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