Brown told senators on Friday he spread a warning in the top echelons at the White House that "we were realising our worst nightmare".
He said he dealt directly with White House officials the day of the 29 August storm, but officials of the Homeland Security Department also were getting regular briefings.
Those he was dealing with, Brown said, included President George Bush's chief of staff, Andrew Card, and deputy chief of staff Joe Hagin.
Administration officials have said they did not realise the severity of Katrina's damage until after the storm had passed.
Brown's testimony bolsters Democratic claims that the Bush
administration ignored signs that a catastrophe had been
The administration's lackluster response to Katrina undermined Americans' confidence in Bush's leadership abilities and contributed directly to a decline in his opinion poll ratings.
Resigned under fire
Brown, in his second appearance in Congress since Katrina, told his side to the senators five months after he resigned under fire as chief of the Federal Emergency Management Agency.
Some senators suggested Brown look inward before pointing the finger elsewhere. "You're not prepared to put a mirror in front of your face and recognise your own inadequacies," said Norm Coleman, a Republican senator.
"Perhaps you may get a more sympathetic hearing if you had a willingness to confess your own sins in this."
Democrats say the government
ignored disaster warnings
Brown responded: "That's very easy for you to say sitting behind that dais and not being there in the middle of that disaster watching that human suffering and watching those people dying and trying to deal with those structural dysfunctionalities, even within the federal government."
The disjointed federal response, Brown said, was in part the result of FEMA being swallowed in 2003 by the newly created Homeland Security Department, which he said was focused on fighting terrorism.
Natural disasters "had become the stepchild of the Department of Homeland Security", he said. Had there been a report that "a terrorist had blown up the 17th Street Canal levee, then everybody would have jumped all over that," he added.