AP

EVEN by the standards of the worst financial crisis for at least a generation, the events of Sunday September 14th and the day before were extraordinary. The weekend began with hopes that a deal could be struck, with or without government backing, to save Lehman Brothers, America's fourth-largest investment bank. Early Monday morning Lehman filed for Chapter 11 bankruptcy protection. It has more than $613 billion of debt.

Other vulnerable financial giants scrambled to sell themselves or raise enough capital to stave off a similar fate. Merrill Lynch, the third-biggest investment bank, sold itself to Bank of America (BofA), an erstwhile Lehman suitor, in a $50 billion all-stock deal. American International Group (AIG) brought forward a potentially life-saving overhaul and went cap-in-hand to the Federal Reserve. But its shares also slumped on Monday.

The situation remains fluid, and investors stampeded towards the relative safety of American Treasury bonds. Stockmarkets tumbled around the world (though some Asian bourses were closed) and the oil price plummeted to well under $100 a barrel. The dollar fell sharply, and the yield on two-year Treasury notes fell below 2% on hopes the Federal Reserve would cut interest rates at a scheduled meeting on Tuesday. American stock futures were deep in the red too. Spreads on risky credit, already elevated, widened further.

With these developments the crisis is entering a new and extremely dangerous phase. If Lehman's assets are dumped in a liquidation, prices of like assets on other firms' books will also have to be marked down, eroding their capital bases. The government's refusal to help with a bail-out of Lehman will strip many firms of the benefit of being thought too big to fail, raising their borrowing costs. Lehman's demise highlights the industry's inability, or unwillingness, to rescue the sick, even when the consequences of inaction are potentially dire.

The biggest worry is the effect on derivatives markets, particularly the giant one for credit-default swaps. Lehman is a top-ten counterparty in CDSs, holding contracts with a notional value of almost $800 billion. On Sunday, banks called in their derivatives traders to assess their exposures to Lehman and work on mitigating risks. The Securities and Exchange Commission, Lehman's main regulator, said it is working with the bank to protect clients and trading partners and to “maintain orderly markets”.

Government officials believed they had persuaded a consortium of Wall Street firms to back a new vehicle that would take $40 billion-70 billion of dodgy assets off Lehman's books, thereby facilitating a takeover of the remainder. But the deal died when the main suitors, BofA and Barclays, a British bank, walked away on Sunday afternoon. Both were unwilling to buy the firm, even shorn of the worst bits, without some sort of government backstop.

But Hank Paulson, the treasury secretary, decided to draw a line and refuse such help. After the Fed had bailed out Bear Stearns in March and the Treasury had taken over Fannie Mae and Freddie Mac last weekend, expectations were high that they would do the same for Lehman. And that was precisely the problem: it would have confirmed that the federal government stood behind all risk-taking in the financial system, creating moral hazard that would take years to undo and expanding taxpayers' liability almost without limit. Conceivably, Congress could have denied Mr Paulson the money he needed even if he had been inclined to bail Lehman out.

This left Lehman with no option but to prepare for bankruptcy. Though the bank has access to a Fed lending facility, introduced after Bear's takeover by JP Morgan Chase, the collapse of its share price left it unable to raise new equity and facing crippling downgrades from rating agencies. Moreover, rival firms that had continued to trade with it in recent weeks—at the urging of regulators—had begun to pull away in the past few days. The inability to find a buyer is a huge blow to Lehman's 25,000 employees, who own a third of the company's now-worthless stock; in such a difficult environment, most will struggle to find work at other financial firms. It also makes for an ignominious end to the career of Dick Fuld, Lehman's boss since 1994, who until last year was viewed as one of Wall Street's smartest managers.

Merrill's rush to sell itself was motivated by fear that it might be next to be caught in the stampede. Despite selling a big dollop of its most rotten assets recently, the market continued to question its viability. Its shares fell by 36% last week, and hedge funds had started to move their business elsewhere. Its boss, John Thain, concluded that it needed to strike a deal before markets reopened. It approached several firms, including BofA and Morgan Stanley, but only BofA felt able to conduct the necessary due diligence in time.

Not only has Mr Thain managed to shelter his firm from the storm, but he has also secured a price well above its closing price last Friday, $29 per share compared with $17. How he managed that in such an ugly market is not yet clear. Ken Lewis, BofA's boss, is no fan of investment banking, but he is a consummate opportunist, and he has coveted Merrill's formidable retail brokerage. Still, the deal carries risks. It will be a logistical challenge, all the more so since BofA is in the middle of digesting Countrywide, a big mortgage lender. Commercial-bank takeovers of investment banks have a horrible history because of the stark cultural differences. And it is not clear if BofA has a clear picture of Merrill's remaining troubled assets.

The takeover of Merrill leaves just two large independent investment banks in America, Morgan Stanley and Goldman Sachs. Both are in better shape than their erstwhile rivals. But this weekend's events cast a shadow over the standalone model, with its reliance on leverage and skittish wholesale funding. Spreads on both banks' CDSs, which reflect investors views of the probability of default, soared on Monday.

Wall Street has company in its misery. Washington Mutual, a big thrift, is fighting for survival under a new boss. Even more worryingly, so is AIG, America's largest insurer, thanks to a reckless foray into CDSs of mortgage-linked collateralised-debt obligations. Investors have fled, fearing the firm will need a lot more new capital than the $20 billion raised so far. Prompted by the weekend bloodletting, AIG brought forward to Monday a restructuring that was to have been unveiled on September 25th. This was expected to include the sale of its aircraft-leasing arm and other businesses. It is also reported to be seeking a $40 billion in bridge loan from the Fed, to be repaid once the sales go through, in the hope that this will attract new capital, possibly from private-equity firms.

With Lehman left dangling, official attention is now turning to putting more safeguards in place to soften the coming shock to markets and the economy. The first step has been to encourage Lehman's counterparties to get together and try to net out as many contracts as possible. On Sunday the Fed also expanded the list of collateral it will accept for loans at its discount window, to include even equities; and dealers may lend any investment-grade security, not just triple-A rated, to the Fed in exchange for Treasury bonds.

Markets are also pricing in some possibility that the Fed will cut its short-term interest rate target from 2% when it meets for a regularly scheduled meeting on Tuesday. That would be an abrupt turnaround from August, when officials figured their next move would be to raise rates, not lower them.

In a sign of how bad things are, even straitened banks are stumping up cash to help the stabilisation efforts. On Sunday, a group of ten banks and securities firms set up a $70 billion loan facility that any of the founding members can tap if it finds itself short of cash.

Even if markets can be stabilised this week, the pain is far from over—and could yet spread. Worldwide credit-related losses by financial institutions now top $500 billion, of which only $350 billion of equity has been replenished. This $150 billion gap, leveraged 14.5 times (the average gearing for the industry), translates to a $2 trillion reduction in liquidity. Hence the severe shortage of credit and predictions of worse to come.

Indeed, most analysts think that the deleveraging still has far to go. Some question how much has taken place. Bianco Research notes that while the credit positions of the 20 largest banks have fallen by $300 billion, to $1.3 trillion, since the Fed started its special lending facilities, the same amount has been financed by the Fed itself through these windows. In other words, instead of deleveraging, the banks have just shifted a chunk of their risk to the central bank. As spectacular as this weekend was, more drama is on the way.