The FBI knew about specific suspected al Qaeda fundraisers before the Sept. 11 attacks but failed to tackle the problem, the September 11 Commission said in a staff report released on Saturday.
In a broad critique of the government's surveillance of terrorist financing, the report said "gaps appear to remain in the intelligence community's understanding of the issue."
The report said the government should resist creating "a terrorist financing czar" or some other specialised entity to focus on the problem. Instead, it backed an existing interagency committee led by the National Security Council.
"The total elimination of money flowing to al Qaeda ... is virtually impossible," the report said.
But it added that efforts to detect and disrupt terrorist money flows are important to limiting "al Qaeda's ability to plan and mount significant mass casualty attacks."
The commission - created to investigate the Sept. 11, 2001, attacks - last month issued a final report that found widespread failings in intelligence. Two staff reports released on Saturday delved into narrower issues, such as financing.
The 9/11 plot cost al Qaeda $400,000 to $500,000, of which about $300,000 was deposited into US bank accounts controlled by the 19 hijackers, the report said.
Al Qaeda funded the hijackers using cash, travellers checks, wire transfers and credit cards in transactions so unremarkable that they largely went unnoticed, it said.
Perceptions that Osama bin Laden personally bankrolled the attacks are incorrect, it said, concluding he had no access to significant personal wealth just before the attacks.
"Rather, al Qaeda relied on diversions from Islamic charities and on well-placed financial facilitators who gathered money from both witting and unwitting donors, primarily in the Gulf region," the report said.
FBI street agents had intelligence about specific suspected fund-raisers before the attacks, but the agency "did not systematically gather and analyse the information its agents developed", the report said.
"The FBI as an organisation failed to understand the nature and extent of the problem or to develop a coherent strategy for confronting it. As a result the FBI could not fulfill its role to provide intelligence on domestic terrorist financing to government policymakers and did not contribute to national policy co-ordination," the report found.
Moreover, it said, the Department of Justice had little success before the attacks developing criminal cases against suspected terrorist fund-raisers, despite a 1996 law that dramatically expanded its power to do so.
Since the attacks, the FBI, the CIA and other intelligence and law enforcement agencies have improved co-ordination and moved effectively to limit the funds available to al Qaeda and use financial information as an investigative tool, it said.
The other staff report issued on Saturday detailed how the hijackers obtained US visas and official documents, such as driver's licenses, some of which they used as identification when they boarded the aircraft on Sept. 11.
The report said some of the hijackers lied on their visa applications, overstayed US visas or falsified their passports.
It said the suspected leader of the hijackers, Mohammed Atta, entered the United States for the last time on July 19, 2001, at Hartsfield International Airport in Atlanta.