WASHINGTON -- Sen. Elizabeth Warren (D-Mass.) raised the stakes of her quest to find out why a single Wall Street bank has not been prosecuted in the aftermath of the financial crisis Tuesday, sending a letter to the heads of three federal agencies.

Warren, a member of the Senate Committee on Banking, Housing & Urban Affairs asked Attorney General Eric Holder, current Securities and Exchange Commission Chairwoman Mary Jo White and Federal Reserve Chairman Ben Bernanke whether they had done any cost-benefit research into prosecuting a bank versus settling with one, which is equivalent to a slap on the wrist for a profitable financial institution.

"Have you conducted any internal research or analysis on trade-offs to the public between settling an enforcement action without admission of guilt and going forward with litigation as necessary to obtain such admission and, if so, can you provide that analysis to my office?" Warren said in the letter.

On Feb. 14, Warren came to her first banking committee hearing and asked federal agencies tasked with bank regulation a related, straightforward question: When was the last time you took a Wall Street bank to trial?

"We do not have to bring people to trial," said Thomas Curry, the head of the Office of the Comptroller of the Currency, the independent bank regulator within the Treasury Department.

Warren then put the question to Elisse Walter, the former SEC chairwoman. Her response: "I will have to get back to you with specific information."

"There are district attorneys and United States attorneys out there every day squeezing ordinary citizens on sometimes very thin grounds and taking them to trial in order to make an example, as they put it. I'm really concerned that 'too big to fail' has become 'too big for trial,'" Warren said.

Warren submitted the question to the OCC for the record, and they responded last week that no, they had not conducted research into trade-offs. "The OCC does not have any internal research or analysis on the trade-offs of settling without an admission of liability," the OCC responded, according to Warren's letter.

Now it's up to Bernanke, Holder and White to answer.

Read the letter here.