NEW ORLEANS — The floodwaters began to drain from this crippled city on Tuesday, and a handful of pumps came fitfully back into operation. But with growing concerns about gas leaks, fires, toxic water and diseases spread by mosquitoes, Mayor C. Ray Nagin said he wanted to ratchet up pressure on the estimated 5,000 to 10,000 remaining residents to leave.

Late Tuesday night the mayor authorized law enforcement officers and the U.S. military to force the evacuation of all residents who refuse to leave the city, saying he did not want the possibility of explosions and disease to increase a death toll that, according to Lt. David Benelli, president of the Police Association of New Orleans, could range from 2,000 to 20,000.

The receding waters were expected to reveal ever more bodies, to be identified by a team of forensic pathologists, medical examiners, coroners and morticians from local funeral homes. "We are going to take one deceased victim at a time and count one at a time," said Robert Johannessen , a spokesman for Louisiana's Department of Health and Hospitals. Of the process of identifying the bodies, he said: "It could take days, it could take years, it could take lifetimes."

The official death toll in Louisiana stood at 83, but state officials said the counting had only begun. In Mississippi, Gov. Haley Barbour announced Tuesday evening that the state's "unofficial but credible estimate" of the death toll was now at 196 but that it was still rising. Barbour said that more than a quarter of the deaths were reported in the state's inland counties, not along the coast.

In Washington, President Bush promised an investigation into what went wrong in the response to Hurricane Katrina and dispatched Vice President Dick Cheney to the Gulf Coast to cut through any bureaucratic obstacles slowing the recovery. The Senate and House also announced their own investigation into the government's response, with the lead Republican senator calling the response "woefully inadequate."

"If our system did such a poor job when there was no enemy, how would the federal, state and local governments have coped with a terrorist attack that provided no advance warning and that was intent on causing as much death and destruction as possible?" Sen. Susan Collins, R-Maine, said in Washington as the Senate Homeland Security Committee prepared for public hearings next week.

Officials said about 60 percent of New Orleans was still under water, but that was down from a peak of about 80 percent. Most of the gain came because the Army Corps of Engineers began opening gaps in the city's levees after the water level in surrounding bodies of water fell. The holes ensured that the levees — designed to keep water out of the below sea-level city — would not hold it in.