A JPMorgan Chase bank branch in Manhattan. Credit: Karsten Moran for The New York Times

The nation’s top bank regulators have added an unexpected voice to the growing chorus of critics worried that the biggest American banks, nearly eight years after the financial crisis, are still too big to fail.

The Federal Reserve and the Federal Deposit Insurance Corporation said on Wednesday that five of the nation’s eight largest banks — including JPMorgan Chase and Bank of America — did not have “credible” plans for how they would wind themselves down in a crisis without sowing panic.

That suggests that if there were another crisis today, the government would need to prop up the largest banks if it wanted to avoid financial chaos.

The announcement coincides with a presidential campaign that at times has been dominated by a debate over what danger the big banks still pose to the nation’s economic security. Senator Bernie Sanders of Vermont has called for the biggest banks to be broken up, a stand that his opponent, the front-runner for the Democratic presidential nomination, Hillary Clinton, has criticized.

But Mr. Sanders’ position has drawn sympathy from some on the other side of the political spectrum, including the new president of the Federal Reserve Bank of Minneapolis, Neel Kashkari, who was a Treasury official during the financial crisis.

In long letters sent to the banks this week, the two regulatory agencies pointed to the dangers created by the global reach and complexity of the largest banks, which are bigger now than they were before the 2008 crisis.

The current arrangement could “pose serious adverse effects to the financial stability of the United States,” the regulators said in their letter to JPMorgan.

The regulators this week did not come close to calling for a breakup, yet they did in effect provide backing for Mr. Sanders’s premise that not enough has been done to safeguard the financial system.

“The goal to end too big to fail and protect the American taxpayer by ending bailouts remains just that: only a goal,” Thomas M. Hoenig, the vice chairman of the F.D.I.C., said in a statement.

The regulators were responding to the so-called living wills that banks must submit to regulators on a regular basis to explain how the banks plan to enter bankruptcy in an orderly fashion in case of a crisis. The living wills are a requirement of the 2010 Dodd-Frank financial overhaul, intended to help make large financial institutions less of a threat to the wider economy.

The Fed and the F.D.I.C., which jointly oversee the largest banks, agreed that the plans put forward by five of the big banks, JPMorgan, Bank of America, Wells Fargo, State Street and Bank of New York Mellon, were “not credible or would not facilitate an orderly resolution under the U.S. Bankruptcy Code.”

Only one of the biggest banks, Citigroup, was given a passing grade by both agencies, though it too was told that its plans needed improvements. Goldman Sachs and Morgan Stanley received passing grades from only one of the two agencies.

Barney Frank, the former Massachusetts congressman who was one of the architects of the Dodd-Frank law, said on Wednesday that the rejection of the living wills indicated that the regulators believed they still have more to do to tackle the too-big-to-fail issue. But he added that the regulators were showing resolve.

“It sends a message,” Mr. Frank said in an interview. “It signals that they are serious.”

In recent weeks, Mr. Frank has criticized the financial overhaul proposals by Mr. Sanders. Mr. Frank contends that some of the plans to break up the banks are simplistic because they take a one-size-fits-all approach. In contrast, he said, measures like the living wills allow regulators to press for banks to shrink, based on specific conditions at those firms.