The US Senate has held a hearing which marks the start of an investigation into the government's response to Hurricane Katrina.
Collins said the response was "sluggish" and lacked coordination
A senior Republican Senator said disaster-response reforms introduced after the 11 September 2001 attacks had failed their first major test.
Meanwhile, US President George W Bush thanked the 115 countries which offered help in the wake of the storm.
He was speaking on the opening day of the United Nations summit in New York.
The hearing came a day after Mr Bush acknowledged the inadequacy of the federal response to the storm and took responsibility for its shortcomings.
Senate Homeland Security Committee chairwoman Susan Collins, a Republican, said although billions of dollars had been used to boost security measures in the wake of the 2001 attack on the US, the response to Katrina had been marred by confusion and lack of co-ordination.
"At this point we would have expected a sharp, crisp response to this terrible tragedy," she said.
"Instead, we witnessed what appeared to be a sluggish initial response."
Congress has not yet decided whether to create an independent commission to investigate the matter or leave the task to a House-Senate panel.
A number of former city and state officials whose communities have suffered major disasters in the past were due to address the panel.
Meanwhile, lawmakers are discussing benefit proposals for Katrina survivors.
Democrats and Republicans are at loggerheads over proposals to temporarily ease restrictive welfare rules to help those made homeless by the storm.
Democrats are in favour of a more generous approach, but Republicans accuse them of trying to take advantage of the emergency to pass ambitious laws which they say would never be approved by the Republican-dominated Congress under normal circumstances.
In a separate development on Tuesday, the owners of a nursing home outside New Orleans were charged with manslaughter over the deaths of 34 people during Hurricane Katrina.
The married couple who ran St Rita's home are accused of ignoring mandatory orders to evacuate residents, whose bodies were found last week.
These are believed to be the first criminal negligence charges brought in connection with the flood disaster.