Attorney General John Ashcroft, former Attorney General Janet Reno and former FBI Director Louis Freeh are scheduled to appear before the panel on Tuesday. FBI Director Robert Mueller will testify on Wednesday, along with CIA Director George Tenet.
They are all expected to face tough questioning about what their agencies were doing to identify and stop al Qaeda operatives known to be in the United States and how high a priority terrorism was in the Clinton and Bush administrations.
The FBI and Justice Department are also central to the debate over whether an intelligence memo sent to President Bush a month before the attacks gave signs of the plot.
Mr. Bush insisted Monday there was no warning in the Presidential Daily Brief of Aug. 6, 2001 that "something is about to happen in America." But he said U.S. intelligence services may be due for reforms.
"There was nothing in there that said, you know, `There is an imminent attack,'" Mr. Bush told reporters. "That wasn't what the report said. The report was kind of a history of Osama's (bin Laden's) intentions."
Democrats have suggested there was more to the memo, the center of an election-year skirmish over the president's anti-terrorism policies before the Sept. 11 attacks. He said he would answer more questions at an East Room news conference Tuesday night. It will be the first formal news conference of the year.
The memo said that since 1998, the FBI had observed "patterns of suspicious activity in this country consistent with preparations for hijackings or other types of attacks." It said the FBI had 70 full field investigations under way.
"The most important feature of the PDB ... is the line that the FBI is conducting 70 full field investigations," former Sen. Slade Gorton, R-Wash., a commission member, said on Fox. "I don't know where those 70 full field investigations were."
Mr. Bush's national security adviser, Condoleezza Rice, talked about such investigations Thursday in her high-profile appearance before the panel. She came under fire from commissioner Tim Roemer, a former Democratic congressman from Indiana.
"We have done thousands of interviews here at the 9-11 commission. We've gone through literally millions of pieces of paper. To date, we have found nobody, nobody at the FBI, who knows anything about a tasking of field offices," Roemer said.
Senior law enforcement officials said the 70 represented every case the bureau was handling that related to terrorism, including even financial crimes and terror groups outside al Qaeda.
Roemer said the acting FBI director at the time, Tom Pickard, testified he had not issued specific new directives regarding the al Qaeda threat and special agents in charge of field offices testified they had no knowledge of one.
Freeh defended the bureau's efforts to combat terrorism before the 2001 attacks in an opinion piece published in Monday's editions of The Wall Street Journal, saying that "short of total war'' the FBI did what it could given the budget and manpower it had to work with at the time.
"Pre-9/11, the FBI used all the means at its disposal to capture (Osama) bin Laden and to prevent future attacks against America,'' Freeh wrote.
"We have now seen how war is declared and waged against terrorists who attack our nation," he added. "The painful lesson is that fighting terrorism without such a declaration of war is unlikely to be successful."
Freeh, who was FBI director from 1993 to 2001, said that al Qaeda and several previous terrorist attacks were among the topics discussed at his first meeting with President Bush and Vice President Cheney on Jan. 26, 2001. Freeh noted that terrorism and al Qaeda had not been an issue in the 2000 presidential campaign, a sign that the nation's attention was elsewhere.
The FBI's counterterrorism budget also reflected those pre-Sept. 11 priorities, Freeh said. For example, he said the FBI asked for 1,895 special agents, analysts and linguists in budget requests for fiscal years 2000 through 2002.
"We got 76 people for those critical years," Freeh said.
In the weeks after Sept. 11, Congress hurriedly approved money for 823 counterterrorism positions and the numbers have steadily climbed since then.
"The al Qaeda threat was the same on Sept. 10 and Sept. 12," Freeh said. "Nothing focuses a government quicker than a war."