Condoleezza Rice has defended her decision not to give public testimony to a congressional commission investigating the 9/11 terror attacks.
Ms Rice - President George Bush's National Security Adviser - said she had "nothing to hide" and was simply following protocol.
The bipartisan commission has made repeated calls for Ms Rice to testify.
A Republican commissioner, John Lehman, has called her refusal to appear "a political blunder of the first order".
Correspondents say the issue threatens to become another political embarrassment for the president.
Ms Rice's role as security adviser is under scrutiny
Ms Rice is considered a key witness for the commission, which is looking into policy against the threat from militants before 11 September 2001, and the response afterwards.
Speaking to CBS TV's 60 Minutes, Ms Rice said it was a "long-standing principle that sitting National Security Advisers do not testify before the Congress".
She said she had spent "about four hours" answering the commission's questions in private.
"We have nothing to hide," she said.
Ms Rice also said she would like to meet the relatives of those killed in the 11 September 2001 attacks "so that I can answer their questions".
But that offer failed to mollify some relatives, who continued to demand a public testimony.
"I'd love to meet with [Ms Rice] as long as it's under oath and it's live in front of television cameras," said Kristen Breitweiser, whose husband Ronald died in New York's World Trade Center.
Ms Rice should "come out and explain what [she] knew, didn't know, what kind of information was passed to the president" and what was not, Lorie Van Auken, widow of another World Trade Center victim, said.
Last week, a former security expert to the White House, Richard Clarke, said the Bush administration had underestimated the terrorist threat because of a fixation on Iraq.
He said President Bush had pressed him to find a link with Saddam Hussein's regime in the hours and days following the attacks.
In the CBS interview, Ms Rice conceded that the president has asked about a link with Iraq.
She said: "The president asked a perfectly logical question - we had just been hit and hit hard - 'did Iraq have anything to do with this, were they complicit?'...
"But when we went to Camp David to plan our response... it was a map of Afghanistan that was rolled out on the table."