Trump’s businesses—maybe because of his fondness for shiny deals—have been the subject of investigations over the years but have not been discussed much in the context of the Trump campaign’s relationship to Russia. But that seems to be shifting. Senator Lindsey Graham, whose committee is heading one of the investigations, raised the question at a hearing on Monday, and apparently asked the White House for information about ties between the President and Russia. In response, lawyers for Trump released a letter to the Associated Press on Friday, saying they had reviewed ten years of Trump’s taxes and didn’t find “any income of any type from Russian sources,” except for a property sold to a Russian billionaire and proceeds from the 2013 Miss Universe pageant held in Moscow. Trump’s actual tax returns weren’t released, so the information could not be confirmed. More significant for the long term, perhaps, was another request made by the Senate Select Committee on Intelligence to the U.S. Treasury Department’s Financial Crimes Enforcement Network, which is known as FinCEN, to turn over documents related to Trump and his campaign officials as part of what Senator Mark Warner, the ranking Democrat on the committee, told CNN is “our effort to try to follow the intel no matter where it leads.”

The FinCEN request is particularly interesting because the unit enforces money-laundering laws and is familiar with Donald Trump’s holdings, specifically the Taj Mahal casino, in Atlantic City. Trump opened the Taj Mahal in 1990. He sold half of his shares in 2004, as part of a bankruptcy settlement, but remained a minority owner. In 2015, the Taj Mahal admitted to “willfully” violating the law by letting many suspicious transactions go unreported to the authorities, and agreed to pay a ten-million-dollar fine—one of the largest ever for a casino. While the fine came at a time when Trump was no longer a majority owner, FinCEN made clear in its public statement that the casino had violations dating back to 2003, when Trump was majority owner, and had faced another fine in 1998. The casino closed late last year.

Casinos can make it remarkably easy to allow people, like drug dealers or corrupt oligarchs, to use funds they obtained illegally. One method is to walk into a casino in a jurisdiction with light regulation— Macau is a favorite—hand several million dollars to a cashier, and ask for a “marker,” or a slip of paper promising repayment. That marker can be transferred to another casino in a different country, where the original depositor or an associate can pick up the millions, play some games, lose some money, and then turn in the remaining chips for cash. With a compliant or unobservant casino, that money can be reported as gambling winnings to the I.R.S. and deposited into a U.S. bank with minimal questioning.