Sen. Elizabeth Warren grilled Federal Reserve Chairwoman Janet Yellen Tuesday on an important provision of the law intended to prevent banks from becoming too big to fail, illuminating the ambiguous status of regulators' attempts to defend against the financial crisis.

At the end of Yellen's semi-annual monetary policy testimony before the Senate Banking Committee, the Massachusetts Democrat asked Yellen about a provision of the 2010 Dodd-Frank financial reform law that requires big banks to submit "living wills" to regulators spelling out in advance how they would be safely resolved in the case of a crisis without triggering a system-wide panic.

If the Fed determines that a bank's living will is not credible, according to the law, it has the power to force the bank to revise and resubmit its plan, raise capital standards, and in the last-case scenario even force it to sell off assets — that is, break it up.

Warren, a consistent critic of Wall Street, noted that the largest U.S. bank, JPMorgan, now has four times the assets that the investment bank Lehman Bros. did when it failed in 2008, exacerbating the financial crisis. Warren also said that JPMorgan has 3,391 subsidiaries, three times as many as Lehman had when its failure threatened to drag down the U.S. economy.

Referring to the living will that the bank has submitted to regulators, Warren asked Yellen if she could "honestly say that JPMorgan could be resolved in an orderly fashion."

Yellen mostly dodged the question of whether the Fed thought JPMorgan's living will was credible. "I understood this to be an iterative process," she responded, saying that the Fed had given the bank "feedback" on the first living will it had submitted and was working on feedback for the second year's submission.

Warren pressed, saying that the law required regulators to make a determination about the credibility of the plans and take action if they did not view them as sufficient — something that neither the Fed nor the Federal Deposit Insurance Corporation has done.

Yellen responded that the Fed tries to give the banks a "roadmap to where we see obstacles to orderly resolution," rather than fully accept or reject the plans, noting that the plans are "extraordinarily complex," running into the tens of thousands of pages.

Warren countered that "the language in the statute is clear,” and implored Yellen to make a determination about the credibility of the living wills it receives, and to use the tools at the Fed's disposal to address the danger posed by a bank without a plan for failure.

The exchange demonstrated that Yellen is skillful at avoiding an unwelcome line of questioning, as her predecessors have been. But Warren succeeded nonetheless in gaining at least an indirect admission that Yellen and other regulators have not settled the too-big-to-fail question as definitively as the Dodd-Frank law envisioned.