At times, as in the case of Southwest, it was as simple as plugging in a number higher than the real one. More frequently, the tactics were akin to illegally claiming a tax deduction by misrepresenting a family vacation as a business expense.

But the claims the Government is examining at Columbia do not involve just a few hundred dollars in air fare and hotel bills -- the amounts stretch into the billions. With so much money at stake, Columbia, once considered almost invincible by competitors, has been shaken to its roots, abandoning both its longtime management team and its aggressive growth strategy.

Few details of the Government's vast inquiry, involving hundreds of agents nationwide, have been publicly disclosed. Just three midlevel executives have been indicted on fraud charges relating to cost reporting at a single Florida hospital. Columbia has begun but not completed an internal inquiry.

The New York Times, in its own nearly two-year investigation of Columbia, has recently obtained confidential records kept by Columbia hospitals in five states. The records show for the first time that certain hospitals camouflaged costs that were not reimbursable and then requested that Government health programs pick up the tab. After the hospitals were sure the trick had gone undetected for two to three years and the risk of a full audit had passed, the money could be counted as profits.

Details are revealed in shadow cost reports and related documents, which often present a cost picture quite different from the one provided to the Government. There is nothing illegal in keeping the second set of records, and many hospitals do so. Medicare rules are so complex, with so many gray areas for interpretation, that hospitals set money aside in case their reading of the regulations is later overruled by auditors. But it is illegal to use such records to set aside money to back cost claims that the hospitals know are not allowable.