An intelligence analyst named Eitan Arusy [at the district attorney's office in Manhattan] began studying a slim lead. Suspicious money was flowing to and from an Iranian nonprofit. Mr. Arusy's probe, later merged with a Justice Department inquiry, ultimately widened to some of Europe's vaunted banks, helping spark a global inquiry that found they actively evaded U.S. law in aiding sanctioned countries, banks or other enterprises move some $2 billion undetected. Nine banks have been caught up in the probe. These weren't rogue operations. The investigators discovered that the banks ran dedicated units to systematically aid the undetected transfer of money through the U.S. banking system. They did that by removing identifying coding on fund transfers so they could evade automated U.S. bank computer systems designed to spot money flowing from a sanctioned state. The far-reaching inquiry started small. Mr. Arusy arrived at the district attorney's office in 2005 to help ferret out illegal financing tied to the Middle East. Though the office prosecutes everyday crime, it carved out a role infiltrating crimes tied to the city's financial markets and institutions. Its expertise dates to the 1990s, when it led the investigation of Bank of Credit & Commerce International, or BCCI, which collapsed in a fraud and money-laundering scandal.