SINGAPORE — Malaysia’s new leader is moving aggressively to investigate the apparent theft of billions of dollars from a state investment fund under the previous government, including seeking the arrest of a key figure in the scandal, the financier Jho Low.

The United States Department of Justice estimates that $4.5 billion went missing from the fund, 1 Malaysia Development Berhad, known as 1MDB, which was established and overseen by the former prime minister Najib Razak — including $731 million that it says was deposited into Mr. Najib’s own bank accounts.

Mr. Najib, who denies any wrongdoing, suffered a surprise election defeat last month at the hands of Mahathir Mohamad, 92, who had previously served more than two decades as prime minister before retiring at 78.

Back in office, Mr. Mahathir has made investigating the scandal and recovering the money a top priority.