The White House is crippling a Senate inquiry into the US government's response to Hurricane Katrina, senators leading the investigation have said.
A report ahead of Katrina warned flood defences could be breached
Democrat Joseph Lieberman, a member of the Senate panel, said warnings about the risk Katrina posed to New Orleans had been ignored.
He accused the White House of being unwilling to hand over documents which might explain why no action was taken.
A White House spokesman insisted the administration was co-operating fully.
Homeland Security Committee senators said agency officials had refused to answer questions about times and dates of meetings and telephone calls with the White House.
"No-one believes the government acted adequately," Senator Lieberman said.
"We can't put a story together if people feel they are under a gag order from the White House."
Some say the response was slow because those affected were poor
The committee's Republican chairwoman, Sen Susan Collins, echoed his criticism of the government.
While some of the president's communications were covered by executive privilege, the administration had gone too far in restricting information about who phoned whom on what day, she said.
Sen Collins has previously criticised the initial response to Katrina as "sluggish" and unco-ordinated.
A White House spokesman said the administration was committed to working with investigators but it had a responsibility to protect the confidentiality of the president's advisers.
"The ability to get advice from advisers on a confidential basis is a critical need for any US president and that is continuing to influence how we cooperate with the committees," spokesman Trent Duffy said.
The BBC's James Coomarasamy in Washington says the final report, which will be published in March, is likely to make uncomfortable reading for the administration.