The Federal Reserve's inspector general has reopened a 2012 case involving leaked information from the central bank and a criminal investigation is pending, according to a letter sent to the Fed by a U.S. Congressman.

House Financial Services Committee Chairman Jeb Hensarling said the Fed's Office of Inspector General (OIG) reopened the case on March 4, according to the letter, which was sent to Fed Chair Janet Yellen on Friday.

Details of the Federal Open Market Committee's September 2012 meeting were published by Medley Global Advisors in a report to clients on Oct. 3 of that year, one day before the Fed released minutes of the gathering.

Hensarling's letter, obtained by Reuters, said he was writing to Yellen "out of concern for your lack of response" to a Feb. 5 letter from Wisconsin Congressman Sean Duffy and requested more information, such as "a list of all individuals with access to the information."

"OIG staff have since informed Committee staff that the case was reopened on March 4, 2015 and an open criminal investigation is currently pending," the letter says.

Bloomberg News was first to report news about the Hensarling letter. Senators ElizabethWarren and Orrin Hatch have also asked Yellen about the matter.

"It is my understanding that although the Federal Reserve's General Counsel was initially involved in this investigation, the inquiry was dropped at the request of several members of the FOMC," Hensarling said in the letter.

The Federal Reserve and the Fed's Office of Inspector General were also not immediately available for comment.