Sen. Susan Collins, a Maine Republican who heads the Senate Governmental Affairs Committee, and Sen. Joseph Lieberman of Connecticut, the panel's top Democrat, said they plan to begin an oversight investigation next week when the full Senate returns from a summer recess.

"We intend to demand answers as to how this immense failure occurred, but our immediate focus must and will be on what Congress can do to help the rescue and emergency operations that are ongoing," the senators said in a joint statement.

"It is also our responsibility to investigate the lack of preparedness and inadequate response to this terrible storm," they said, adding that it was "increasingly clear that serious shortcomings in preparedness and response have hampered relief efforts at a critical time."

Senate Majority Leader Bill Frist, a Tennessee Republican, called for the investigation saying he hoped the lessons learned would improve the government's response to future disasters.

The Bush administration's handling of the disaster that wreaked havoc in the Gulf Coast and spilled a devastating flood into New Orleans has come under sharp criticism.

As President George W. Bush toured the disaster area, Louisiana Democratic Sen. Mary Landrieu said the Federal Emergency Management Agency, which the Bush administration placed under the Department of Homeland Security, failed to deploy enough resources to the area quickly.

She called on Bush to appoint a Cabinet-level official to direct the national response to the calamity.

"There was a time when FEMA understood that the correct approach to a crisis was to deploy to the affected area as many resources as possible as fast as possible," Landrieu said. "Unfortunately that no longer seems to be their approach."

Congress sent Bush a $10.5 billion emergency spending bill on Friday to cover some of the initial costs of the recovery effort and lawmakers have promised much more.