LOS ANGELES (MarketWatch) -- U.S. law-enforcement officials have launched an investigation into possible collusion among U.S. and European banks to fix the London interbank offered rate, or Libor, according to a Wall Street Journal report citing unidentified people familiar with the situation. The report said the probe, conducted over the past year, is looking at whether the banks acted in concert to understate their borrowing costs, used to calculate Libor, during 2006-2008. The report said officials had sent subpoenas to Bank of America Corp. BAC, +1.34% , Citigroup Inc. C, +1.62% and UBS AG UBS, +1.27% among others.