New York - A U.S. government watchdog group has asked two federal oversight agencies to investigate billionaire Sheldon Adelson to determine if any money he donated to political campaigns in the past was illegally laundered in his foreign casinos.

Adelson, the single biggest Republican donor in the 2012 presidential election, is the chief executive of Las Vegas Sands Corp, which derives the bulk of its revenue from casinos in the global gaming hot spot of Macau, an hour’s ferry ride from Hong Kong.

The complaints, drawn up by the Campaign for Accountability, were filed earlier this month with the Federal Election Commission and the U.S. Senate Committee on Homeland Security and Governmental Affairs.

Both agencies declined to comment. The FEC has the ability to launch investigations into campaign finance issues, including ones relating to foreign money, but has historically been mired in partisan gridlock.

Las Vegas Sands spokesman Ron Reese dismissed the complaints by the Campaign for Accountability as “a politically motivated political attack.” The group, which has lodged complaints against both Republicans and Democrats, describes itself as nonpartisan and nonprofit but conservative organizations say it has a left-wing agenda.

The filings refer to internal Las Vegas Sands Corp documents, which have surfaced as part of an ongoing lawsuit brought by the former CEO of Adelson’s operation in Macau, Steven Jacobs, who is alleging wrongful termination.

According to the documents, which Reuters has reviewed, the company had a business relationship with Cheung Chi Tai, who U.S. authorities have identified as a leader of a Chinese organized crime ring, or triad. Cheung is awaiting trial in Hong Kong on charges of money laundering.

In its complaint, the group asserts that, given how much Adelson’s wealth derives from Macau, “it is quite likely Macau organized crime funds and foreign money have wound up in the coffers of candidates for federal office and/or in the treasuries of dark money groups supporting them.”

Reese, the Las Vegas Sands spokesman, acknowledged the company conducted business with Cheung, who guaranteed the debts of gamblers, but said it severed the relationship in 2010 after the company became aware of his background. “The company has not done any business with Cheung Chi Tai in many years,” he said.

Adelson, the 15th-richest man in America according Forbes, poured more than $100 million into the 2012 election to support Republican candidates Newt Gingrich and Mitt Romney. He is expected to be a major donor in the November 2016 presidential election although he has yet to endorse a candidate.