US investigators are examining whether some of the world's biggest banks colluded to manipulate a key interest rate before and during the financial crisis, affecting trillions of dollars in loans and derivatives, say people familiar with the situation.

For the past year, law-enforcement officials have been investigating whether the US and European banks understated their own borrowing costs, which are used to calculate the London interbank offered rate, or Libor. The investigators are now looking into whether the banks effectively formed a global cartel and coordinated how to report borrowing costs between 2006 and 2008.