CLSA analyst Mike Mayo said his support for Wells Fargo & Co. Chief Executive John Stumpf was "wavering" following problematic sales tactics that has brought him under fire from U.S. lawmakers.

The bank has been facing heightened scrutiny following allegations that its employees had set up as many as 2 million accounts and credit cards in customers' names that may have not been authorized.

Mayo said his support for Stumpf was contingent upon the CEO's ability to better explain the repercussions of the sales abuses.

Last week, lawmakers on the U.S. Senate Banking Committee grilled Stumpf about the accounts, with some calling on him to resign and forfeit his earnings and hold other senior executives accountable.

Shareholders have a right to know what will be done to the pay of top executives, to the composition of the board, and for impacted customers, Mayo said in a note to clients.

"We believe Wells Fargo is bigger than the CEO, notwithstanding a good financial track record during his tenure, and there should be no more excuses for the lack of answers to key questions," Mayo said.

Mayo rates the stock 'outperform' with a price target of $57. The stock's median price target is $51.

Shares of the San Francisco, California-based lender have fallen by about 10 percent since Sept. 7, the day before the bank reached a $185-million settlement over fraud case.