In a letter sent Tuesday to Powell, Warren criticized him for having multiple private meetings and talks with top executives — including 19 with JPMorgan Chase & Co. CEO Jamie Dimon since February 2018. 

Warren wrote that these meetings and conversations between Powell and the executives "appear to be shaping Fed policy" and that the regulator is now "floating a gutted proposal," referring to the bank-capital plan. 

Read more: Federal Reserve Floats Weaker Version of Bank-Capital Revamp

The Massachusetts Democrat asked Powell a series of questions, including whether he had made any commitments to Dimon or other big-bank executives related to the capital overhaul, which is tied to Basel III, an international agreement that followed the 2008 financial crisis. 

Warren also requested information on any discussions about bank-merger guidelines and executive-pay reform. She asked for a list of the topics covered in all of Powell's meetings with Dimon. 

"We have received the letter and plan to respond," the central bank said in a statement Tuesday.

In May, the Wall Street Journal reported that Dimon had pushed his peers to avoid Fed Vice Chair for Supervision Michael Barr, seen as the architect of the bank-capital plan, and instead lobby Powell and other Fed governors about changing the proposal. Shortly after, Warren sent a separate letter accusing Powell of "doing Mr. Dimon's bidding."

Warren is a member of the Senate Banking Committee where Powell is set to testify next week. 

In March, the central bank chief told lawmakers that he expected "broad and material changes" to the proposal that American regulators released last July. That original plan could have forced the US's eight largest banks to set aside about 19% more in capital.