Earlier, the commission reported that the CIA lacked the funding or a plan to improve the way it collected and analyzed intelligence on terrorism before the 2001 attacks.
Tenet insisted he was working to improve his agency when the terrorists struck, and that "the warning was well understood" on al Qaeda's danger.
But he acknowledged, "We made mistakes."
In its second day of hearings into warnings of the attacks and the federal response, the report by the commission staffed blamed "too many priorities, declining attention to the craft of strategic analysis, budget constraints, sophisticated denial and deception efforts by adversaries, and security rules" for flaws in CIA analysis on terrorism.
The critique, which, indicated the agency was slow to recognize the threat of Osama bin Laden's terrorist network. Al Qaeda was formed in 1988, but the intelligence community did not document its rise until 1999.
As the bipartisan panel, weighed whether to revamp the U.S. intelligence community, the report said a key question framed by the pre-Sept. 11 failures was: "Who is in charge of intelligence?"
Among those scheduled to testify Wednesday were Tenet, FBI Director Robert Mueller and officials with the Homeland Security Department and the Terrorist Threat Integration Center.
The staff report criticized not only how the CIA analyzed intelligence, but also how it collected and processed it.
"There was not a comprehensive review of what the (intelligence) community knew, what it did not know," the report found.
Weaknesses in analysis were especially evident in the warnings the CIA and did not issue — concerning, for example, the possibility that hijacked jets might be used as weapons.
For years there were signs that terrorists might be contemplating such a plot. In 1994, a small plane crashed into the White House lawn, and Algerian terrorists tried to fly a plane into the Eiffel Tower but were unable to pilot the craft.
In 1995, a detained terrorist told Philippines investigators that he considered flying a plane into CIA headquarters. The next year, there was a report that Iranians considered crashing a hijacked plane in Tel Aviv.
In 1998, there was an uncorroborated report of a plot to crash aircraft into a U.S. city, and a foreign government heard word of a plot to hijack a plane to gain the released of terrorists held by the United States.
Commissioner Richard Ben-Veniste said in the months before Sept. 11, NORAD conducted a simulation in which a commercial aircraft slammed into the Pentagon.
However, the CIA's counterterrorism center "did not analyze how a hijacked aircraft or other explosives-laden aircraft might be used as a weapon"
It "did not develop a set of tell-tale indicators for this means of attack. For example, once such indicator might be the discovery of terrorist seeking or taking flight training to fly large jet aircraft, or seeking to buy advance flight simulators."
An August 2001 report to top CIA officials titled "Islamic Extremist Learns to Fly," detailing the arrest of Zacarias Moussaoui, "had no evident effect on warning," the commission reported.
Tenet said he was briefed on Moussaoui on Aug. 23 or 24, 2001, but did not see the president once in August.
Tenet saw the flaws in terrorism analysis in 2000 and appointed a senior official to correct them. Adding ten staff positions to the counterterrorism center "was seen as a major bureaucratic victory."
Commissioner John Lehman called the report "damning."
Tenet, however, contended that he did have a plan for improving the CIA's abilities by increasing the number of agents and strengthening links within the intelligence community.
"The intelligence that we provided our senior policymakers…was clear and direct," Tenet said. "The warning was well understood even if the timing and method of attacks."
"However, we never penetrated the 9/11 plot overseas," he said. "We made mistakes."
He pointed to the failure to track Khalid al-Mihdhar and Nawaf al-Hazmi, two suspected terrorists who were not added to a watch-list before the entered the country. They were among the 19 hijackers on Sept. 11.
For example, he said there were four separate terrorist identity databases and dozens of terrorist watch-lists.
"No matter how hard we worked or how desperately we tried, it was not enough," Tenet warned, estimating that, "It will take us another five years to have the kind of clandestine service our country needs."
He attributed that delay, in part, to the need to train new agents.
Addressing possible changes to the community structure, Tenet warned against any separation between intelligence collectors and analysts. He said that the official responsible for providing intelligence to the president must have knowledge of any covert actions that are under way.
"You can't build this community in fits and starts," Tenet said.
He said he opposes the CIA taking over the domestic intelligence work done by the FBI because of concerns about U.S. privacy laws.
"We operate with a certain degree of impunity to other country's laws," Tenet said. That impunity could not be maintained inside the United States, he said.