That subpoena, which the complaint said Ng did not respond to, stemmed from a probe in Nevada involving the Foreign Corrupt Practices Act (FCPA), which prohibits bribing foreign officials, said the source, who spoke on condition of anonymity to discuss the case.
The target of that investigation was unclear. But Ng's name has repeatedly surfaced in a private lawsuit against billionaire Sheldon Adelson's Las Vegas Sands Corp that the casino operator has said it believed prompted an FCPA investigation.
Kevin Tung, Ng's lawyer, said on Thursday he did not know what the subpoena said and was still gathering information. The U.S. Justice Department declined comment. Las Vegas Sands in a statement said it is cooperating with the investigation.
Ng, 68, heads Macau-based Sun Kian Yip Group, and has made much of what U.S. prosecutors called his $1.8 billion fortune on lavish developments in the former Portuguese colony.
His properties include the Fortuna casino and residential complex Windsor Arch, which overlooks Macau's race course and the Cotai Strip.
Ng also sits on the Chinese People's Political Consultative Conference, an adviser to the government.
He and his assistant Jeff Yin were arrested on Saturday for allegedly falsely claiming $4.5 million in cash they brought into the United States from China between 2013 to 2015 was meant to buy art, antiques or real estate, or be used for gambling.
At a hearing on Saturday, U.S. prosecutor Daniel Richenthal said Yin told authorities after his arrest that the money was used to pay people "to engage in unlawful activities," according to a transcript.[ID:nL1N11S1U7]
Yin's lawyer, Sabrina Shroff, disputed that account, and Tung in an interview said the case was a "misunderstanding."
Ng's name previously surfaced in U.S. investigations into how foreign money might have been funneled into the Democratic National Committee prior to the 1996 elections, when it was working to re-elect President Bill Clinton.
A 1998 U.S. Senate report said that Ng collaborated with Little Rock, Arkansas restaurateur Charlie Tries to funnel hundreds of thousands of dollars in foreign funds to the DNA.
Tries later pleaded guilty. Nag, who prosecutors say stopped coming to the United States for five years amid that probe, was never charged.
The 2014 subpoena cited in the complaint came after Nag's name first surfaced in a lawsuit filed in 2010 against Las Vegas Sands by the former CEO of Adel son's Macau casinos, Steven Jacobs.
Macau is the only legal casino hub in China, and Sands China Ltd, a publicly traded unit of Las Vegas Sands, is one of six licensed casino operators in the southern Chinese territory.
Las Vegas Sands has said it believes that FCPA investigations by the Justice Department and U.S. Securities and Exchange Commission were prompted by Jacobs' lawsuit.
Jacobs contended he was fired after resisting Adel son's demands that Las Vegas Sands rehire a lawyer, Leone Elves, who was also a Macau legislator and who allegedly told the company Chinese officials had offered to resolve some issues facing it for a $300 million payment.
Nag allegedly was Elves' contact with the officials, acting as what a lawyer for Jacobs described in a court hearing in May as a "courier or messenger."
No evidence exists the payment was ever made, and the company denies wrongdoing. Testifying in May, Adel son said he did not know Nag and had never sent any messages to him or heard of his name until Jacobs' lawsuit.
Lawyers for Jacobs did not respond to requests for comment.
The case is U.S. v. Sen, U.S. District Court, District of New York, No. 15-mj-03369.
Follow us on Twitter: @CNBCWorld