WASHINGTON—Timothy Geithner in 2008 sent a private memo to Bank of England Governor Mervyn King calling for six changes that he said would improve the credibility and integrity of the London interbank offered rate, a key interest rate that is now at the center of a international banking scandal, according to documents reviewed by the Wall Street Journal.

At the time the memo was sent, Mr. Geithner was president of the Federal Reserve Bank of New York and the financial industry was about to enter one of the darkest periods of the financial crisis. Mr. Geithner is now U.S. Treasury secretary. As Mr. Geithner sent the memo to London, U.S. regulators also began conferring about concerns related to possible distortions of Libor and what the impact might be, people familiar with the matter said.

The June 2008 memo, reported earlier by the Washington Post, provides a window into the role played by U.S. regulators in the Libor scandal, though possibly an incomplete window. More documents are due to be released Friday by the Federal Reserve Bank of New York, in response to demands by lawmakers for more information about Mr. Geithner's and the New York Fed's efforts to address questions about Libor.

The latest disclosure makes clear that Fed officials were aware of irregularities in the Libor interest-rate market. What is less clear is how far Mr. Geithner and other officials went to address the problem.

The Geithner recommendations, which came in a June 1, 2008, memo, included a call to "eliminate incentive to misreport" by banks. Investigators in the U.S. and U.K. are now probing whether banks intentionally misreported interbank lending rates in a way that distorted Libor, which could have affected interest rates for trillions of dollars-worth of financial banking products all over the world, including mortgages, student loans and complex derivatives. The misreporting of this interest rate also could have given the public and regulators a false sense of the health of the big banks involved in this market.