United States investigators have launched probes into corruption allegations that hundreds of millions of dollars linked to a Malaysian state fund had dropped into prime minister Najib Razak’s personal bank accounts.

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A federal grand jury is examining the claims involving Najib and people close to him, the New York Times said, citing two people with knowledge of the investigation.

The inquiry, run by the US Justice Department’s international corruption unit, is focused on properties in the United States purchased by companies that belong to Najb’s stepson as well as the transfer of $681m into what is believed to be Najib’s personal bank account.

The Wall Street Journal also reported the Federal Bureau of Investigation had launched an investigation into allegations of money-laundering at the debt-laden Malaysian state fund 1MDB, citing an unidentified source close to the matter.

The Malaysian premier is fighting his way through the worst political crisis of his career. He has denied the allegations but also sacked four ministers, his attorney general and deputy prime minister in a bid to disarm his critics.

Authorities have blocked two news publications and a British-based whistleblowing website run by Gordon Brown’s sister-in-law. A former senior member of Najib’s ruling party said he was prevented on Friday from travelling to New York where he planned to lodge a complaint with US police against 1MDB.

Despite the crackdown, the investigation has managed to snowball outside the country’s borders. Hong Kong police are now investigating bank deposits purported to be linked to Najib. Swiss authorities have frozen millions of dollars in bank accounts linked to 1MDB.

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Ex-prime minister, Mahathir Mohamad, an influential leader who ruled Malaysia for 22 years, joined protests in the capital last month which called for Najib to step down. Many Malaysians are already angry following the implementation of a goods and services tax this year as well as a devaluation in the local ringgit currency to a 17-year low.

On Monday, Malaysia’s central bank governor said the public has the right to know about the findings of the investigation into 1MDB. “Right now, we know that the public wants answers to these questions, and they deserve to get the answers,” she was quoted as saying by local media.

On the same day, the Malaysian high court ruled the two publications the home minister shut down in July for their reporting on 1MDB, would be allowed to resume publication on Tuesday.