Rescuing the nation’s mortgage giants, Fannie Mae and Freddie Mac, could well cost American taxpayers hundreds of billions of dollars and quite possibly more than $5 trillion. Fixing the mess is essential, but so is finding and punishing the people who got us into it. Don’t worry about Monday morning quarterbacking. We need accountability as well as repairs to the system.

Despite reassurances by administration officials and some leading members of Congress that the two huge mortgage organizations can survive, Wall Street has shown its concern by slaughtering their market values. Among others, former St. Louis Federal Reserve President William Poole told Bloomberg news that they were already “insolvent.”

The Bush administration, in its Sunday night rescue plan, responded to the downhill stock market rather than the cheery statements that all would be well. In an effort to restore the collapsing financial confidence, the plan would put federal support behind the mortgage companies, up to $300 billion – for starters. Congress would have to approve and could act as early as this week.

The big unknown, of course, is how long and how deep will be the collapse of the housing market and the mortgage crisis. No one knows, although Treasury Secretary Henry M. Paulson Jr., in soothing terms, called it “the current housing correction.” Mortgage defaults could continue for several years. With many other lenders scared off, Fannie Mae and Freddie Mac hold or guarantee half of the nations $12 trillion mortgage total. If they should fail, the loss to home buyers and investors here and abroad would be incalculable.

Fannie Mae began in the 1930s as a government-sponsored mortgage backup for troubled banks and soon was joined by Freddie Mac. They are owned by shareholders. Without any formal federal guarantee of their solvency, they nonetheless enjoyed the fruits of a widespread assumption that the backup was there if needed. That led to perks like borrowing at low rates, lavish salaries and bonuses for its managers, and freedom from any serious federal supervision or control.

Both companies profited enormously from the housing boom, in which homeowners and speculators generally assumed that values would keep on rising indefinitely. It all ended with the rude shock of the burst bubble.

Who should be held responsible for the mess? Partly, of course, the gullible home buyers who borrowed beyond their means and often contracted for future rate increases.

But the lenders who talked them into unwise mortgages are more to blame. Behind them all was the swelling loan hoard of Fannie and Freddie, whose managers were reaping huge salaries and protecting themselves by hiring public officials and their relatives, building an army of lobbyists and making generous political contributions to both parties. Congress brushed aside efforts to stem the headlong rush toward the present crunch.

Congress can make amends for its past complicity by acting swiftly to shore up and regulate the mortgage giants and by exposing the misdeeds that brought us to this point.