WASHINGTON — Top financial regulators are defending their decision to rescue Bear Stearns from collapse last month, but under intense and skeptical grilling from lawmakers they could offer no assurances that the extraordinary move would be the only taxpayer-backed bailout of a major Wall Street bank.

In congressional testimony yesterday, the officials said they took the best option available to them when they offered up to $30 billion in government funds to back the sale of Bear Stearns to JPMorgan Chase, insisting that the potential failure of Bear Stearns could have brought down other major firms as well.

“We judged that a sudden, disorderly failure of Bear would have brought with it unpredictable but severe consequences for the functioning of the broader financial system and the broader economy,” the president of the Federal Reserve Bank of New York, Timothy Geithner, told members of the Senate Banking Committee on Capitol Hill.

Yesterday’s hearing amounted to a public post-mortem on the rescue of Bear Stearns, as federal officials and executives from the two banks recounted a round-the-clock flurry of conference calls and meetings that precipitated the takeover of Bear Stearns three weeks ago.

While the chairman of the Securities and Exchange Commission, Christopher Cox, said regulators had been monitoring Bear Stearns on a daily basis since 2007, its rapid deterioration in value March 13 — caused by customers scrambling to withdraw cash and lenders refusing to extend credit — caught officials by surprise.

The firm’s chief executive, Alan Schwartz, blamed the collapse on the lightning-quick spread of “unfounded rumors and attendant speculation” that Bear Stearns was in the midst of a liquidity crisis. The rumors, he said, “became a self-fulfilling prophecy.”

“There was, simply put, a run on the bank,” Mr. Schwartz said.

Federal officials said they offered the $30 billion in public backing once JPMorgan concluded it could not buy the firm on its own without great risk to its shareholders. “This wasn’t a negotiating posture. It was the plain truth,” the chief executive of JPMorgan, Jamie Dimon, said. The bank agreed to assume the first $1 billion in losses if the deal does not earn a profit. “We went absolutely as far as we could go,” he said.

Lawmakers from both sides of the aisle pressed the regulators — occasionally in exasperated tones — to explain how they could be caught off-guard by the Bear Stearns collapse and what they were doing to prevent defaults by other major financial institutions. “There were an awful lot of red flags, not just in the last six weeks, not just in the last six months,” Senator Bunning, a Republican of Kentucky, said.

“Nobody was watching the store. So it was eventually going to happen. It just happened to be Bear Stearns,” he added.

A Democrat of New Jersey, Senator Menendez, similarly scolded the panel. “I think what we have here is a clean-up brigade,” he said, “not a protector of all the institution that we need protected for the well being of all Americans.”

The chairman of the Federal Reserve, Ben Bernanke, acknowledged that the Fed’s action constituted a “bailout,” but he characterized it as a bailout of the economy and not Bear Stearns investors. “If you wanted to say we bailed out the market in general, I guess that’s true,” he said.

He expressed confidence, but could not guarantee, that taxpayers would get most of their $30 billion back. “We believe we will recover most or all of it. Probably all of it,” Mr. Bernanke said.

Mr. Geithner stressed that the threat of future instability was not over, and he called for regulatory changes that established better safeguards to ensure greater liquidity for financial firms and a reduced risk of sudden failure.

Officials said they recognized that the Fed’s move raised “moral hazard” concerns, but they sought to underscore that Bear Stearns investors were not in an enviable position and that the risk of inaction to the broader economy outweighed the worry that the rescue would encourage dangerous betting.

Still, the committee’s Democratic chairman, Senator Dodd of Connecticut, urged the SEC to investigate claims of short-selling in advance of the Bear Stearns collapse, pointing out that investors who bought $600 worth of stock before the deal could have made out with $37,000 just days later.

“Your hopes will be, I think, met and exceeded,” Mr. Cox replied, though he avoided discussing an ongoing inquiry. “The rumors,” he added, “are too big to miss.”