The Federal Reserve Bank of New York in 2012 studied whether a key foreign-exchange benchmark could be subject to manipulation, but it didn't take any public action as a result, according to people familiar with the matter.

Today, that same benchmark-the so-called WM/Reuters fix-is at the center of a burgeoning investigation into whether bank traders and others colluded to manipulate the $5.3-trillion-a-day currencies market. Roughly two dozen bank traders have been fired or suspended. Banks are bracing for potentially huge civil and criminal penalties. Until earlier this year, the Fed has been absent from the long list of authorities publicly probing the situation.