The US inquiry, which people familiar with the matter said is likely in its early stages, is also seeking to establish if there was any retaliation against whistle-blowers, according to the subpoena.

Marina Bay Sands is one of the most profitable casinos in the world, accounting for more than one-fifth of revenue and about a third of operating income at the US parent. Las Vegas Sands’ Asian operations, which also include Macau, contributed about 85 per cent of the company’s US$13.7 billion in revenue last year, and have helped make Adelson one of the richest men in the US.

A spokeswoman for the US Attorney’s Office in Las Vegas said she could not confirm or deny ongoing investigations. The subpoena also requested information on another former casino employee, who people familiar said carried out fund transfers to high rollers.

In a written response, the Singapore casino said any suggestion of inappropriate activity is taken seriously, and it has investigated every assertion of wrongdoing brought to its attention. Marina Bay Sands and its parent company haven’t received any requests from the Department of Justice, according to people familiar with the matter.

Las Vegas Sands has drawn other Justice Department scrutiny in recent years. In 2013, the company paid US$47.4 million to end a federal investigation into its acknowledged failure to report suspicious deposits by a high-stakes gambler in Las Vegas.

In 2017, it paid US$6.96 million to resolve a probe into alleged violations of US law in connection with bribes paid to government officials in China and Macau. Sands admitted it knowingly and willingly failed to ensure the legitimacy of about US$5.8 million in payments to a business consultant.

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Marina Bay Sands also faces a probe in Singapore by the Casino Regulatory Authority into its money transfer policies. A spokesperson for the agency declined to comment further as the investigation is ongoing, adding it hasn’t received a request from the Department of Justice in connection with Marina Bay Sands.

The regulator also said it’s “committed to ensuring that the casinos in Singapore, including Marina Bay Sands, remain free from criminal influence or exploitation, and takes a serious view of any allegations of unauthorised money transfers”.

Claims about these transfers surfaced in a lawsuit filed last year by Wang Xi, who sued Marina Bay Sands seeking to recover S$9.1 million (US$6.5 million) that he said was sent to other casino patrons in 2015 without his approval. The Singapore Police Force is also investigating Wang’s complaint. The casino has declined to comment on the suit.

The Singapore regulator asked Marina Bay Sands to review its third-party transfer process, one of the people said. Such transfers, when authorised, are legal and used by groups of gamblers to share winnings and losses at different foreign casinos.

These transfers are sometimes made through so-called junket operators, which provide transport, hotels and credit to high rollers. In Macau, these operators allow Chinese gamblers to get around strict capital controls by pledging assets on the mainland in exchange for credit at casinos. The junkets are more strictly controlled in Singapore.

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Marina Bay’s internal probe found instances of its group employees violating accepted transfer procedures by filling in payment details on pre-signed or photocopied authorisation forms, according to a person familiar with the matter. It also uncovered cases in which original documents were destroyed, the person said.

Such practices appear to have stopped since April 2018, when the casino – which has had at least six chief compliance officers in the last decade – amended its procedures, according to the person familiar.

“When allegations related to the mishandling of ‘letters of authorisation’ were made, the company thoroughly reviewed the matter,” a Marina Bay Sands spokesman said in the statement. “The review concluded that no patron funds were transferred in a manner that was contrary to a patron’s intent.”

The company has shared this information with the authorities in Singapore and will cooperate with any governmental requests it may receive, according to the statement.

Singapore requires casinos to implement internal controls and check the authorisation of fund transfers, as well as comply with requirements to prevent money laundering and terrorism financing, the regulator said.

Marina Bay Sands is one of two firms granted licences to operate casinos on the island. Last year, the Singapore government agreed to extend the licences held by Genting Singapore and Las Vegas Sands to 2030, in exchange for pledges to invest a combined S$9 billion in tourism projects.

With a profit margin ranging from 53 per cent to 56 per cent in the three years ending in 2019, Marina Bay Sands is among the most profitable integrated gaming resorts in the world.

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Like many casinos around the world, Marina Bay Sands is closed due to the pandemic, slashing revenue for firms like Las Vegas Sands. Macau gaming revenue across all casinos plunged 93 per cent in May from the year earlier, as operators await the opening of borders to spark a recovery after an unprecedented shutdown.