The U.S. Supreme Court on Monday ruled against the government in two money laundering cases, making it more difficult for prosecutors to use an important weapon in the war on drugs and organized crime.

In a unanimous decision, Justice Clarence Thomas said that a money laundering case cannot be proven merely by showing that funds were concealed while being transported.

In a 5-4 ruling, Justice Antonin Scalia said that money laundering refers to profits of an illegal operation, not gross receipts. The court's interpretation is a narrow one opposed by law enforcement agencies.

Money laundering carries a maximum penalty of 20 years in prison and prosecutors often use it in drug and gambling cases.

Congress enacted the anti-money laundering law in 1986 after the President's Commission on Organized Crime highlighted the growing problem of "washing" criminal proceeds through overseas bank accounts and legitimate businesses.