National Security Advisor Condoleezza Rice is testifying publicly and under oath to the independent commission investigating the September 11, 2001 terrorist attacks on the United States.
In her opening statements Thursday, Ms. Rice said in the 20 years prior to the September 11 attacks, America's response to terrorism had been insufficient. She says the attacks put the United States on war footing.
She also said the Bush administration understood that al-Qaida posed a serious threat to the United States before September 11.
During her two-and-a-half hour sworn testimony, Ms. Rice is expected to be questioned by the 10-member panel on allegations from former counterterrorism chief Richard Clarke, who says the Bush administration did not make terrorism an urgent priority before the September 11 attacks.
Ms. Rice is also expected to dispute Mr. Clarke's assertions that the administration made Iraq too much of a priority after the attacks.
Ms. Rice's appearance is being broadcast live by the major U.S. television networks and overseas by Voice of America. The National Security Advisor has already met privately with the commission, but that was before the allegations by Mr. Clarke.
The commission's chairman, Republican Thomas Kean, has said he wants Ms. Rice to clear up what he calls "discrepancies" in some of her public statements about the September 11 attacks.
President Bush had long resisted calls to have Ms. Rice appear publicly before the commission. But he reversed course last week amid mounting bipartisan pressure.
President Bush and Vice President Cheney have agreed to meet jointly and in private with the panel. Mr. Bush said earlier this week he looks forward to sharing information with the commission. He also defended his record, saying he immediately went to war against al-Qaida after the September 11 attacks.