When Deputy Attorney General Rod Rosenstein announced he was handing over the reins of the Justice Department's Russia investigation to a special counsel, he gave Robert Mueller the authority to look into "any matters that arose or may arise directly from the investigation."
It has become clear in the time since that Mueller intends to use that clause to follow the money.
The former FBI director has assembled a team of lawyers, many of whom specialize in white collar law and financial crimes, leading to speculation that he is diving into the finances of those on the inner and outer orbit of the Trump campaign.
"They're looking for ties, they're looking for relationships, and a lot of that will come down to money trails," said Jennifer Rodgers, the executive director of the Center for the Advancement of Public Integrity at Columbia Law School.
And when it comes to chargeable offenses Mueller could be zeroing in on, money laundering is at the top of the list.
Here's an explainer (jump to specific sections by clicking on the links below):
Put simply: Money laundering turns "dirty" money "clean" — making proceeds from criminal activity usable without drawing the attention of law enforcement.
John Cassara, a money laundering expert who worked for the State Department and the Department of Treasury for more than 25 years, says money launderers work through a three-step system.
1. Placement: This is the first time that money acquired through a crime — stolen, say, or through drug or human trafficking — enters the financial system. The cash can be blended into a legitimate business' cash flow and receipts, or deposited in small amounts to banks for later transfer. This is where launderers are most vulnerable to being caught, says Cassara, as banks and financial institutions have a number of tools to try to catch suspicious activity.
Intel, for instance, just announced new artificial intelligence software that monitors "transactions and suggestive personality behaviors" to catch would-be launderers, according to Axios.
2. Layering: The second step is where a money launderer covers their tracks. By transferring the money between companies and accounts in different places, the goal here is to make it difficult for law enforcement to follow the trail because of jurisdictional issues and transparency laws.
Many countries and even some states require very little personal information to start a company, leading bad actors to start what are called "shell companies" — companies without any purpose other than to be a disguising vehicle for money to change hands.
By the time someone is spending money in the United States, for example, it might have been transferred from Cyprus, having been transferred from Ukraine, having been transferred from, say, Russia, where it might have been stolen or received as payment in the drug or human trafficking trade.
"You can also layer it by switching it from cash to gold to cyber to commodities," Cassara said.
3. Integration: Once the money has been moved, moved and moved again, it's time to legitimize it. Cassara says luxury items, property or stock investments are all options for a criminal to park the rewards of the ill-gotten gains.
For example, there are whole sections of the richest sections of London owned by wealthy foreigners that mostly sit empty, leading locals to wonder whether they're at the end of a pipeline of illicit money. Real estate in hot markets such as London or New York is a good investment for anyone because it seldom loses value and can often be quickly resold in legitimate transactions, freeing up cash. In the U.K., people call this phenomenon "Lights Out London."
"Outside of crimes of passion, for example murder committed in a jealous rage, why do criminals and criminal organizations commit crime?" Cassara asks rhetorically. "Greed."
"Essentially, with any crime that generates money, the criminal needs to launder the money in order to use it," says Stefanie Ostfeld, the deputy head of the U.S. office for Global Witness, a nonprofit focused on exposing economic networks that breed corruption.
"The issue with money laundering is it tends to take two to money launder: You have the criminal who committed the original crime, but then you also have the bank, the lawyer, the accountant that actually moved the money into the financial system."
The United Nations Office on Drugs and Crime estimates that between 2 percent and 5 percent of global gross domestic product is laundered each year. Even if you take the lowest end of that guess, it's around $800 billion.
Ostfeld says the root of the problem is lax laws that make it easy to create businesses, or shell companies, with anonymous or hard-to-trace owners as part of the layering process.
"Time and time again, our investigations highlight the same problem," Ostfeld said. "That's sham companies that allow those who steal state funds to move it undetected through the international financial system."
Places like the island of Jersey and the Cayman Islands have reputations as shell company havens, but an article published earlier this year in The Atlantic found the U.S. to be "perhaps the foremost shell-company provider globally."
Delaware, for instance, is famous around the world for being an easy place to set up a shell company. In 2012, the state had more corporate entities than people, according to The New York Times. And as NPR reported, at least one entity linked with Russian advocates who want the U.S. to roll back sanctions on Moscow is chartered in Delaware.
Over the past 50 years, the U.S. has made policy changes toward curbing money laundering. Notably, Congress passed the Bank Secrecy Act in 1970, which requires banks to report cash transactions over $10,000.
In 1986, the Money Laundering Control Act made money laundering a federal crime, and laws in the 1990s pushed financial institutions to begin filing reports on suspicious activity.
Still, Cassara says despite the 18 million or so pieces of "financial intelligence" that will be filed in 2017, less than 1 percent of the money that is laundered will be recovered.
"To be a money launderer today, you'd have to be either very stupid or very unlucky to get caught," Cassara said. "The magnitude is really high, but the sobering part is that our success rate is staggeringly low."
Ostfeld said that is because despite having tools at their disposal to quell the money's movement, law enforcement and banks have not made it a priority to do so.
"Bankers are doing the minimum they can get away with," Ostfeld said. "What people don't really understand is that corruption doesn't just hurt people 'over there' ... It actually impacts so many aspects of life, we just don't see it because it's one piece of the puzzle.
"But then you take a step back and you realize it's actually what joins all these crimes together."
It's not a secret that Trump and those close to his campaign have had some financial ties to Russia.
"Russians make up a pretty disproportionate cross section of a lot of our assets," as Donald Trump Jr. said at a real estate conference in 2008. "We see a lot of money pouring in from Russia."
And it's also no secret that corruption in Russia means oligarchs there can't always keep money in Russian banks. They move it overseas, to Europe or the United Kingdom or the U.S., for use in buying real estate or other purposes. Often, they have help: One of the Russians who met with Trump Jr. in June 2016 in Trump Tower, Irakly Kaveladze, was said to have set up some 2,000 shell companies in the United States.
"Over the past three decades, at least 13 people with known or alleged links to Russian mobsters or oligarchs have owned, lived in and even run criminal activities out of Trump Tower and other Trump properties," wrote Craig Unger, in a long feature for The New Republic.
"Whatever his knowledge about the source of his wealth, the public record makes clear that Trump built his business empire in no small part with a lot of dirty money from a lot of dirty Russians."
Rodgers of the Center for the Advancement of Public Integrity says it may not be Trump who should necessarily worry about money laundering charges at this point. The president may have good reason to fear, according to some reports, but the person in the most pressing legal jeopardy could be Trump's former campaign manager, Paul Manafort.
McClatchy reported in August that Mueller was "zeroing in" on Manafort's finances.
"I personally think they're looking at money laundering for Manafort," Rodgers said. "It looks like they may get something there, given the sheer number of accounts he has, and the number of transactions involved."
A spokesman for Manafort declined to comment for this story. No one has been charged with any crime so far in the Russia imbroglio, and Trump has repeatedly denied any wrongdoing in connection with Russia.
All the same, observers including Rodgers say Mueller may try to use the threat of financial charges to flip Manafort into being a witness against others in the campaign.
"Anyone facing serious charges for money laundering and tax offenses involving undisclosed foreign bank accounts would have huge incentives to trade cooperation for prison time," Jonathan Winer, the State Department's top money laundering expert during the Clinton administration, told McClatchy.
Details about how deep Manafort's ties go continue to emerge.
NBC reported recently on a $26 million loan between Manafort and Russian oligarch Oleg Deripaska, making the pair's total financial relationship about $60 million over the past decade.
"Money launderers frequently will disguise payments as loans," Stefan Cassella, a former federal prosecutor, told NBC. "You can call it a loan, you can call it 'Mary Jane.' If there's no intent to repay it, then it's not really a loan. It's just a payment."
While there aren't any charges yet, Mueller's prosecutors reportedly told Manafort in September that they plan at some point to indict him.