The Secret Service is the main government agency specifically involved with money crimes, such as counterfeiting and securities fraud. That said, money laundering comes under RICO and thus the FBI, so the FBI getting involved makes sense, but the FBI is not part of treasury.
While it is true they are most known for prodviding security for the president, they also have the job of hunting down people who counterfeit money. job. Actually, they are not part of Tresury anymore either. In 2003 they were moved to the Department of Homeland Security, but they were in the treasury.
The FBI has always been part of the justice department. As my trusty World Book says "The is the investigative arm of the United States department of justice". They work closely with the United States district attorney's offices to inveigate crimes that the district attorneys offices then prosecute.
Of course since this is alt-world, the FBI can be different. Still, in our world the FBI is part of the judtice department.
