Stuart Levey, undersecretary of the U.S. Treasury for terrorism and financial intelligence, told the newspaper cash is much more difficult to track than wire transfers, but also more cumbersome.
"It's a lot less efficient," Levey said. "Some cash couriers get caught. Some get greedy."
He said another dilemma for terrorists was the need to break the cash up into smaller amounts to prevent one weak link from undermining an entire operation.
Meanwhile, the U.S. Immigration and Customs Enforcement agency has been training foreign counterparts to detect and block cash smuggling, ICE spokesman Dean Boyd told USA Today.