"But how do you prove corruption? By its nature, corruption is covert; payoffs are designed to be difficult to detect. The international financial system has evolved to accommodate a wide array of illicit activities, and shell companies and banking havens make it easy to camouflage transfers, payment orders, and copies of checks. Paul Collier argues that there are often three parties to a corrupt deal: the briber, the bribed, and the lawyers and financial facilitators who enable the secret transaction. The result, he says, is “a web of corporate opacity” that is spun largely by wealthy professionals in financial capitals like London and New York. A recent study found that the easiest country in which to establish an untraceable shell company is not a tropical banking haven but the United States."

http://www.newyorker.com/reporting/2013/07/08/130708fa_fact_keefe?currentPage=all