When the Deep fire broke out in the Sequoia National Forest on Aug. 12, 2004, Forest Service fire managers projected the cost of fighting it would be modest -- about $200 an acre.

But costs raced faster than the flames. "The fire brought a significant amount of political pressure" to protect a nearby state park, giant sequoia trees and small mountain towns, a new federal audit says. As a result, firefighters threw "everything but the kitchen sink" at the fire, the audit adds.

Five days later, the Deep fire was out, but containing it had cost a small fortune: $3,000 an acre, or 15 times more than expected. In all, fire managers spent $9 million on the fire -- an average of $1.8 million a day.

The runaway expenses are cited in the recent U.S. Department of Agriculture Office of Inspector General audit, which sharply criticizes Forest Service fire managers for spending excessively, failing to learn from the past and fighting fires that should have been allowed to burn naturally.

Although jolted by the findings, Forest Service officials said they intend to act on them.

"It wasn't the cake and ice cream I might have been hoping for -- but I accept the findings," said Tom Harbour, the agency's director of fire and aviation, who requested the audit. "I'm convinced there is progress to be made in ... cost containment. I'm going to redouble our efforts."

Among the audit's highlights:

* The Forest Service spends enormous sums defending communities along the edges of national forests in the rapidly developing "wildland-urban interface" zone. Up to 90 percent of the cost of fighting large fires is in protecting private property, which drains money from protecting public resources, such as endangered species and water quality.

* "Despite abundant evidence that natural wildfire reduces hazardous fuels," the Forest Service still puts out nearly all fires, in part because it is rooted in "a culture of fire suppression." Unless officials restore wildfire more frequently to the landscape, vegetation will thicken and "environmental damage and suppression costs are certain to escalate."

* The agency's own efforts to control expenditures -- through cost-containment reviews -- have failed because Forest Service managers are not required to correct problems and findings are not distributed widely enough.

The inspector general audit comes in the wake of the nation's costliest fire season. This year, fires began early in Oklahoma and Texas, then moved to the Rockies, the Northwest and California over the summer and fall.

In all, the Forest Service spent more than $1.5 billion putting them out -- including $100 million to $200 million siphoned from nonfire budgets such as reforestation.

The spending has attracted the attention of the White House Office of Management and Budget, the Government Accountability Office and congressional committees.

"For taxpayers to spend up to $2 billion a year suppressing fires with what in many respects is a century-old policy simply does not make sense, economically or environmentally," said Sen. Jeffrey Bingaman, a New Mexico Democrat, in a statement.

The largest factor in rising costs, the audit found, is the expense of protecting the homes and communities that have sprouted around the edges of federal forests.

Nationwide, more than 8.5 million homes popped up in the so-called wildland-urban interface zone in the 1990s, Harbour said. Even though the structures are on private land, the Forest Service rushes to defend them, typically with the help of state and local fire departments.

The march of development into the woods "is an escalating fire problem," the audit says, adding that many "landowners take no action to reduce their homes' vulnerability to wildfire and many local governments do not require" them to do so.

Although the Forest Service has cost-sharing arrangements with other fire departments, the audit says they are not updated often enough and the Forest Service, unable to regulate the development, "bears a disproportionate share of protection costs."

Letting fire perform its natural role of cleansing the forest of debris will reduce costs in the long run, the audit says. But it states: "Existing firefighting policies and the lack of qualified personnel restrict Forest Service managers from doing so.

"Of almost 80,000 natural ignitions that occurred on Forest Service land from 1998 to 2005, only about 1,500, or 2 percent, were allowed to burn ... ."

This summer, for instance, natural fire was allowed to burn 1,500 acres in California, mostly on the Mendocino National Forest, an agency spokesman said.